Context adaptive binary arithmetic coding method and device

ABSTRACT

There is provided an video encoding/decoding method and apparatus. The video decoding method comprises acquiring a bitstream including a predetermined context element, performing at least one of a context model determination, a probability update, and a probability interval determination on the predetermined syntax element, and arithmetically decoding the predetermined syntax element on the basis of a result of the performance.

TECHNICAL FIELD

The present invention relates to a method and apparatus for videocoding/decoding, and a recording medium storing a bitstream. Moreparticularly, the present invention relates to a method and apparatusfor video coding/decoding based on CABAC.

BACKGROUND ART

In video coding, context adaptive binary arithmetic coding (CABAC) isused for entropy coding on occurrence symbols (beans) for many syntaxelements such as prediction information, transform coefficient, andsignaling information. The CABAC can specify occurrence bins for thecorresponding syntax element as a real value for a certain probabilityinterval. In the arithmetic coding, the symbol occurrence probability isupdated to be close to the actual occurrence probability from theinitial probability according to the occurrence state (LPS, MPS) of thesymbol of each syntax. Since only one probability update model is usedin the probability update in the related art, the stability of theprobability update and the convergence rate have a trade-offrelationship. Also, since a probability update model with a fixed tableor parameter is used, it is difficult to appropriately reflect theoccurrence probability that varies depending on the time variable duringarithmetic coding.

DISCLOSURE Technical Problem

The objective of the present invention is to provide a method andapparatus for video coding/decoding with improved compressionefficiency.

In addition, the present invention has an objective to provide a methodand apparatus for video coding/decoding using CABAC with improvedcompression efficiency.

In addition, the present invention has an objective to provide arecording medium storing a bitstream generated by the method orapparatus for video coding/decoding according to the present invention.

Technical Solution

According to an aspect of an embodiment, a video decoding method maycomprise acquiring a bitstream including a predetermined contextelement; performing at least one of a context model determination, aprobability update, and a probability interval determination on thepredetermined syntax element; and arithmetically decoding thepredetermined syntax element on the basis of a result of theperformance.

The context model determination may include at least one of a contextinitialization process, an adaptive context model selection process, anda context model storage/synchronization process.

The context initialization process may be performed in at least one unitof a picture (frame), a slice, a tile, a CTU line, a CTU, a CU, and apredetermined block size.

The adaptive context model selection process may select a context modelon the basis of at least one of prediction information and syntaxelement of a current block and a neighboring block.

The context model storage/synchronization process may be performed in atleast one unit of a picture (frame), a slice, a tile, a CTU line, a CTU,a CU, and a predetermined block size.

The probability update may include at least one of a table-basedprobability update, an operation-based probability update, amomentum-based probability update, a multiple probability update, and aboundary-based probability update.

In the table-based probability update, the probability table may beconstructed by quantizing the probability range to a positive integer.

The momentum-based probability update may be performed on the basis of acurrent occurrence symbol and a past occurrence symbol.

The probability interval determination may include at least one of atable-based probability interval determination and an operation-basedprobability interval determination.

In the table-based probability interval determination, the probabilityinterval table may be represented by a two-dimensional table of anoccurrence probability index (positive integer M) and a currentprobability interval (positive integer N).

According to an aspect of another embodiment, a video coding method maycomprise performing at least one of a context model determination, aprobability update, and a probability interval determination on apredetermined syntax element; arithmetically decoding the predeterminedsyntax element on the basis of a result of the performance; andgenerating a bitstream including the predetermined syntax elementarithmetically decoded.

The context model determination may include at least one of a contextinitialization process, an adaptive context model selection process, anda context model storage/synchronization process.

The context initialization process may be performed in at least one unitof a picture (frame), a slice, a tile, a CTU line, a CTU, a CU, and apredetermined block size.

The adaptive context model selection process may select a context modelon the basis of at least one of prediction information and syntaxelement of a current block and a neighboring block.

The context model storage/synchronization process may be performed in atleast one unit of a picture (frame), a slice, a tile, a CTU line, a CTU,a CU, and a predetermined block size.

The probability update may include at least one of a table-basedprobability update, an operation-based probability update, amomentum-based probability update, a multiple probability update, and aboundary-based probability update.

In the table-based probability update, the probability table may beconstructed by quantizing the probability range to a positive integer.

The momentum-based probability update may be performed on the basis of acurrent occurrence symbol and a past occurrence symbol.

According to an aspect of another embodiment, a computer-readablerecording medium storing a bitstream generated by a video coding methodmay comprise performing at least one of a context model determination, aprobability update, and a probability interval determination on apredetermined syntax element; arithmetically decoding the predeterminedsyntax element on the basis of a result of the performance; andacquiring a bitstream including the predetermined syntax elementarithmetically decoded.

Advantageous Effects

According to the present invention, a method and apparatus for videocoding/decoding with improved compression efficiency can be provided.

Also, according to the present invention, a method and apparatus forvideo coding/decoding using CABAC with improved compression efficiencycan be provided.

Also, according to the present invention, a recording medium storing abitstream generated by the video coding/decoding method or apparatus ofthe present invention can be provided.

DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating a configuration of a codingapparatus according to an embodiment of the present invention.

FIG. 2 is a block diagram illustrating a configuration of a decodingapparatus according to an embodiment of the present invention.

FIG. 3 is a diagram schematically illustrating a partitioning structureof a video when coding and decoding the video.

FIG. 4 is a diagram illustrating an embodiment of an intra predictionprocess.

FIG. 5 is a diagram illustrating an embodiment of an inter predictionprocess.

FIG. 6 is a diagram illustrating transformation and quantizationprocesses.

FIG. 7 is a diagram illustrating reference samples capable of being usedfor intra prediction.

FIG. 8 is a diagram illustrating a unit in which context modelinginitialization is performed according to an embodiment of the presentinvention.

FIG. 9 is a diagram illustrating neighboring blocks of a current blockaccording to an embodiment of the present invention.

FIG. 10 is a flowchart illustrating a video decoding method according toan embodiment of the present invention.

FIG. 11 is a flowchart illustrating a video coding method according toan embodiment of the present invention.

MODE FOR INVENTION

A variety of modifications may be made to the present invention andthere are various embodiments of the present invention, examples ofwhich will now be provided with reference to drawings and described indetail. However, the present invention is not limited thereto, althoughthe exemplary embodiments can be construed as including allmodifications, equivalents, or substitutes in a technical concept and atechnical scope of the present invention. The similar reference numeralsrefer to the same or similar functions in various aspects. In thedrawings, the shapes and dimensions of elements may be exaggerated forclarity. In the following detailed description of the present invention,references are made to the accompanying drawings that show, by way ofillustration, specific embodiments in which the invention may bepracticed. These embodiments are described in sufficient detail toenable those skilled in the art to implement the present disclosure. Itshould be understood that various embodiments of the present disclosure,although different, are not necessarily mutually exclusive. For example,specific features, structures, and characteristics described herein, inconnection with one embodiment, may be implemented within otherembodiments without departing from the spirit and scope of the presentdisclosure. In addition, it should be understood that the location orarrangement of individual elements within each disclosed embodiment maybe modified without departing from the spirit and scope of the presentdisclosure. The following detailed description is, therefore, not to betaken in a limiting sense, and the scope of the present disclosure isdefined only by the appended claims, appropriately interpreted, alongwith the full range of equivalents to what the claims claim.

Terms used in the specification, ‘first’, ‘second’, etc. can be used todescribe various components, but the components are not to be construedas being limited to the terms. The terms are only used to differentiateone component from other components. For example, the ‘first’ componentmay be named the ‘second’ component without departing from the scope ofthe present invention, and the ‘second’ component may also be similarlynamed the ‘first’ component. The term ‘and/or’ includes a combination ofa plurality of items or any one of a plurality of terms.

It will be understood that when an element is simply referred to asbeing ‘connected to’ or ‘coupled to’ another element without being‘directly connected to’ or ‘directly coupled to’ another element in thepresent description, it may be ‘directly connected to’ or ‘directlycoupled to’ another element or be connected to or coupled to anotherelement, having the other element intervening therebetween. In contrast,it should be understood that when an element is referred to as being“directly coupled” or “directly connected” to another element, there areno intervening elements present.

Furthermore, constitutional parts shown in the embodiments of thepresent invention are independently shown so as to representcharacteristic functions different from each other. Thus, it does notmean that each constitutional part is constituted in a constitutionalunit of separated hardware or software. In other words, eachconstitutional part includes each of enumerated constitutional parts forconvenience. Thus, at least two constitutional parts of eachconstitutional part may be combined to form one constitutional part orone constitutional part may be divided into a plurality ofconstitutional parts to perform each function. The embodiment where eachconstitutional part is combined and the embodiment where oneconstitutional part is divided are also included in the scope of thepresent invention, if not departing from the essence of the presentinvention.

The terms used in the present specification are merely used to describeparticular embodiments, and are not intended to limit the presentinvention. An expression used in the singular encompasses the expressionof the plural, unless it has a clearly different meaning in the context.In the present specification, it is to be understood that terms such as“including”, “having”, etc. are intended to indicate the existence ofthe features, numbers, steps, actions, elements, parts, or combinationsthereof disclosed in the specification, and are not intended to precludethe possibility that one or more other features, numbers, steps,actions, elements, parts, or combinations thereof may exist or may beadded. In other words, when a specific element is referred to as being“included”, elements other than the corresponding element are notexcluded, but additional elements may be included in embodiments of thepresent invention or the scope of the present invention.

In addition, some of constituents may not be indispensable constituentsperforming essential functions of the present invention but be selectiveconstituents improving only performance thereof. The present inventionmay be implemented by including only the indispensable constitutionalparts for implementing the essence of the present invention except theconstituents used in improving performance. The structure including onlythe indispensable constituents except the selective constituents used inimproving only performance is also included in the scope of the presentinvention.

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. In describingexemplary embodiments of the present invention, well-known functions orconstructions will not be described in detail since they mayunnecessarily obscure the understanding of the present invention. Thesame constituent elements in the drawings are denoted by the samereference numerals, and a repeated description of the same elements willbe omitted.

Hereinafter, an image may mean a picture configuring a video, or maymean the video itself. For example, “encoding or decoding or both of animage” may mean “encoding or decoding or both of a moving picture”, andmay mean “encoding or decoding or both of one image among images of amoving picture.”

Hereinafter, terms “moving picture” and “video” may be used as the samemeaning and be replaced with each other.

Hereinafter, a target image may be an encoding target image which is atarget of encoding and/or a decoding target image which is a target ofdecoding. Also, a target image may be an input image inputted to anencoding apparatus, and an input image inputted to a decoding apparatus.Here, a target image may have the same meaning with the current image.

Hereinafter, terms “image”, “picture, “frame” and “screen” may be usedas the same meaning and be replaced with each other.

Hereinafter, a target block may be an encoding target block which is atarget of encoding and/or a decoding target block which is a target ofdecoding. Also, a target block may be the current block which is atarget of current encoding and/or decoding. For example, terms “targetblock” and “current block” may be used as the same meaning and bereplaced with each other.

Hereinafter, terms “block” and “unit” may be used as the same meaningand be replaced with each other. Or a “block” may represent a specificunit.

Hereinafter, terms “region” and “segment” may be replaced with eachother.

Hereinafter, a specific signal may be a signal representing a specificblock. For example, an original signal may be a signal representing atarget block. A prediction signal may be a signal representing aprediction block. A residual signal may be a signal representing aresidual block.

In embodiments, each of specific information, data, flag, index, elementand attribute, etc. may have a value. A value of information, data,flag, index, element and attribute equal to “0” may represent a logicalfalse or the first predefined value. In other words, a value “0”, afalse, a logical false and the first predefined value may be replacedwith each other. A value of information, data, flag, index, element andattribute equal to “1” may represent a logical true or the secondpredefined value. In other words, a value “1”, a true, a logical trueand the second predefined value may be replaced with each other.

When a variable i or j is used for representing a column, a row or anindex, a value of i may be an integer equal to or greater than 0, orequal to or greater than 1. That is, the column, the row, the index,etc. may be counted from 0 or may be counted from 1.

Description of Terms

Encoder: means an apparatus performing encoding. That is, means anencoding apparatus.

Decoder: means an apparatus performing decoding. That is, means andecoding apparatus.

Block: is an M×N array of a sample. Herein, M and N may mean positiveintegers, and the block may mean a sample array of a two-dimensionalform. The block may refer to a unit. A current block my mean an encodingtarget block that becomes a target when encoding, or a decoding targetblock that becomes a target when decoding. In addition, the currentblock may be at least one of an encode block, a prediction block, aresidual block, and a transform block.

Sample: is a basic unit constituting a block. It may be expressed as avalue from 0 to 2Bd−1 according to a bit depth (Bd). In the presentinvention, the sample may be used as a meaning of a pixel. That is, asample, a pel, a pixel may have the same meaning with each other.

Unit: may refer to an encoding and decoding unit. When encoding anddecoding an image, the unit may be a region generated by partitioning asingle image. In addition, the unit may mean a subdivided unit when asingle image is partitioned into subdivided units during encoding ordecoding. That is, an image may be partitioned into a plurality ofunits. When encoding and decoding an image, a predetermined process foreach unit may be performed. A single unit may be partitioned intosub-units that have sizes smaller than the size of the unit. Dependingon functions, the unit may mean a block, a macroblock, a coding treeunit, a code tree block, a coding unit, a coding block), a predictionunit, a prediction block, a residual unit), a residual block, atransform unit, a transform block, etc. In addition, in order todistinguish a unit from a block, the unit may include a luma componentblock, a chroma component block associated with the luma componentblock, and a syntax element of each color component block. The unit mayhave various sizes and forms, and particularly, the form of the unit maybe a two-dimensional geometrical figure such as a square shape, arectangular shape, a trapezoid shape, a triangular shape, a pentagonalshape, etc. In addition, unit information may include at least one of aunit type indicating the coding unit, the prediction unit, the transformunit, etc., and a unit size, a unit depth, a sequence of encoding anddecoding of a unit, etc.

Coding Tree Unit: is configured with a single coding tree block of aluma component Y, and two coding tree blocks related to chromacomponents Cb and Cr. In addition, it may mean that including the blocksand a syntax element of each block. Each coding tree unit may bepartitioned by using at least one of a quad-tree partitioning method, abinary-tree partitioning method and ternary-tree partitioning method toconfigure a lower unit such as coding unit, prediction unit, transformunit, etc. It may be used as a term for designating a sample block thatbecomes a process unit when encoding/decoding an image as an inputimage. Here, the quad-tree may mean a quarternary-tree.

When the size of the coding block is within a predetermined range, thedivision is possible using only quad-tree partitioning. Here, thepredetermined range may be defined as at least one of a maximum size anda minimum size of a coding block in which the division is possible usingonly quad-tree partitioning. Information indicating a maximum/minimumsize of a coding block in which quad-tree partitioning is allowed may besignaled through a bitstream, and the information may be signaled in atleast one unit of a sequence, a picture parameter, a tile group, or aslice (segment). Alternatively, the maximum/minimum size of the codingblock may be a fixed size predetermined in the coder/decoder. Forexample, when the size of the coding block corresponds to 256×256 to64×64, the division is possible only using quad-tree partitioning.Alternatively, when the size of the coding block is larger than the sizeof the maximum conversion block, the division is possible only usingquad-tree partitioning. Herein, the block to be divided may be at leastone of a coding block and a transform block. In this case, informationindicating the division of the coded block (for example, split_flag) maybe a flag indicating whether or not to perform the quad-treepartitioning. When the size of the coding block falls within apredetermined range, the division is possible only using binary tree orternary tree partitioning. In this case, the above description of thequad-tree partitioning may be applied to binary tree partitioning orternary tree partitioning in the same manner.

Coding Tree Block: may be used as a term for designating any one of a Ycoding tree block, Cb coding tree block, and Cr coding tree block.

Neighbor Block: may mean a block adjacent to a current block. The blockadjacent to the current block may mean a block that comes into contactwith a boundary of the current block, or a block positioned within apredetermined distance from the current block. The neighbor block maymean a block adjacent to a vertex of the current block. Herein, theblock adjacent to the vertex of the current block may mean a blockvertically adjacent to a neighbor block that is horizontally adjacent tothe current block, or a block horizontally adjacent to a neighbor blockthat is vertically adjacent to the current block.

Reconstructed Neighbor block: may mean a neighbor block adjacent to acurrent block and which has been already spatially/temporally encoded ordecoded. Herein, the reconstructed neighbor block may mean areconstructed neighbor unit. A reconstructed spatial neighbor block maybe a block within a current picture and which has been alreadyreconstructed through encoding or decoding or both. A reconstructedtemporal neighbor block is a block at a corresponding position as thecurrent block of the current picture within a reference image, or aneighbor block thereof.

Unit Depth: may mean a partitioned degree of a unit. In a treestructure, the highest node(Root Node) may correspond to the first unitwhich is not partitioned. Also, the highest node may have the leastdepth value. In this case, the highest node may have a depth of level 0.A node having a depth of level 1 may represent a unit generated bypartitioning once the first unit. A node having a depth of level 2 mayrepresent a unit generated by partitioning twice the first unit. A nodehaving a depth of level n may represent a unit generated by partitioningn-times the first unit. A Leaf Node may be the lowest node and a nodewhich cannot be partitioned further. A depth of a Leaf Node may be themaximum level. For example, a predefined value of the maximum level maybe 3. A depth of a root node may be the lowest and a depth of a leafnode may be the deepest. In addition, when a unit is expressed as a treestructure, a level in which a unit is present may mean a unit depth.

Bitstream: may mean a bitstream including encoding image information.

Parameter Set: corresponds to header information among a configurationwithin a bitstream. At least one of a video parameter set, a sequenceparameter set, a picture parameter set, and an adaptation parameter setmay be included in a parameter set. In addition, a parameter set mayinclude a slice header, a tile group header, and tile headerinformation. The term “tile group” means a group of tiles and has thesame meaning as a slice.

Parsing: may mean determination of a value of a syntax element byperforming entropy decoding, or may mean the entropy decoding itself.

Symbol: may mean at least one of a syntax element, a coding parameter,and a transform coefficient value of an encoding/decoding target unit.In addition, the symbol may mean an entropy encoding target or anentropy decoding result.

Prediction Mode: may be information indicating a mode encoded/decodedwith intra prediction or a mode encoded/decoded with inter prediction.

Prediction Unit: may mean a basic unit when performing prediction suchas inter-prediction, intra-prediction, inter-compensation,intra-compensation, and motion compensation. A single prediction unitmay be partitioned into a plurality of partitions having a smaller size,or may be partitioned into a plurality of lower prediction units. Aplurality of partitions may be a basic unit in performing prediction orcompensation. A partition which is generated by dividing a predictionunit may also be a prediction unit.

Prediction Unit Partition: may mean a form obtained by partitioning aprediction unit.

Reference picture list may refer to a list including one or morereference pictures used for inter prediction or motion compensation.There are several types of usable reference picture lists, including LC(List combined), L0 (List 0), L1 (List 1), L2 (List 2), L3 (List 3).

Inter prediction indicator may refer to a direction of inter prediction(unidirectional prediction, bidirectional prediction, etc.) of a currentblock. Alternatively, it may refer to the number of reference picturesused to generate a prediction block of a current block. Alternatively,it may refer to the number of prediction blocks used at the time ofperforming inter prediction or motion compensation on a current block.

Prediction list utilization flag indicates whether a prediction block isgenerated using at least one reference picture in a specific referencepicture list. An inter prediction indicator can be derived using aprediction list utilization flag, and conversely, a prediction listutilization flag can be derived using an inter prediction indicator. Forexample, when the prediction list utilization flag has a first value ofzero (0), it means that a reference picture in a reference picture listis not used to generate a prediction block. On the other hand, when theprediction list utilization flag has a second value of one (1), it meansthat a reference picture list is used to generate a prediction block.

Reference picture index may refer to an index indicating a specificreference picture in a reference picture list.

Reference picture may mean a reference picture which is referred to by aspecific block for the purposes of inter prediction or motioncompensation of the specific block. Alternatively, the reference picturemay be a picture including a reference block referred to by a currentblock for inter prediction or motion compensation. Hereinafter, theterms “reference picture” and “reference image” have the same meaningand can be interchangeably.

Motion vector may be a two-dimensional vector used for inter predictionor motion compensation. The motion vector may mean an offset between anencoding/decoding target block and a reference block. For example, (mvX,mvY) may represent a motion vector. Here, mvX may represent a horizontalcomponent and mvY may represent a vertical component.

Search range may be a two-dimensional region which is searched toretrieve a motion vector during inter prediction. For example, the sizeof the search range may be M×N. Here, M and N are both integers.

Motion vector candidate may refer to a prediction candidate block or amotion vector of the prediction candidate block when predicting a motionvector. In addition, a motion vector candidate may be included in amotion vector candidate list.

Motion vector candidate list may mean a list composed of one or moremotion vector candidates.

Motion vector candidate index may mean an indicator indicating a motionvector candidate in a motion vector candidate list. Alternatively, itmay be an index of a motion vector predictor.

Motion information may mean information including at least one of theitems including a motion vector, a reference picture index, an interprediction indicator, a prediction list utilization flag, referencepicture list information, a reference picture, a motion vectorcandidate, a motion vector candidate index, a merge candidate, and amerge index.

Merge candidate list may mean a list composed of one or more mergecandidates.

Merge candidate may mean a spatial merge candidate, a temporal mergecandidate, a combined merge candidate, a combined bi-predictive mergecandidate, or a zero merge candidate. The merge candidate may includemotion information such as an inter prediction indicator, a referencepicture index for each list, a motion vector, a prediction listutilization flag, and an inter prediction indicator.

Merge index may mean an indicator indicating a merge candidate in amerge candidate list. Alternatively, the merge index may indicate ablock from which a merge candidate has been derived, among reconstructedblocks spatially/temporally adjacent to a current block. Alternatively,the merge index may indicate at least one piece of motion information ofa merge candidate.

Transform Unit: may mean a basic unit when performing encoding/decodingsuch as transform, inverse-transform, quantization, dequantization,transform coefficient encoding/decoding of a residual signal. A singletransform unit may be partitioned into a plurality of lower-leveltransform units having a smaller size. Here,transformation/inverse-transformation may comprise at least one amongthe first transformation/the first inverse-transformation and the secondtransformation/the second inverse-transformation.

Scaling: may mean a process of multiplying a quantized level by afactor. A transform coefficient may be generated by scaling a quantizedlevel. The scaling also may be referred to as dequantization.

Quantization Parameter: may mean a value used when generating aquantized level using a transform coefficient during quantization. Thequantization parameter also may mean a value used when generating atransform coefficient by scaling a quantized level duringdequantization. The quantization parameter may be a value mapped on aquantization step size.

Delta Quantization Parameter: may mean a difference value between apredicted quantization parameter and a quantization parameter of anencoding/decoding target unit.

Scan: may mean a method of sequencing coefficients within a unit, ablock or a matrix. For example, changing a two-dimensional matrix ofcoefficients into a one-dimensional matrix may be referred to asscanning, and changing a one-dimensional matrix of coefficients into atwo-dimensional matrix may be referred to as scanning or inversescanning.

Transform Coefficient: may mean a coefficient value generated aftertransform is performed in an encoder. It may mean a coefficient valuegenerated after at least one of entropy decoding and dequantization isperformed in a decoder. A quantized level obtained by quantizing atransform coefficient or a residual signal, or a quantized transformcoefficient level also may fall within the meaning of the transformcoefficient.

Quantized Level: may mean a value generated by quantizing a transformcoefficient or a residual signal in an encoder. Alternatively, thequantized level may mean a value that is a dequantization target toundergo dequantization in a decoder. Similarly, a quantized transformcoefficient level that is a result of transform and quantization alsomay fall within the meaning of the quantized level.

Non-zero Transform Coefficient: may mean a transform coefficient havinga value other than zero, or a transform coefficient level or a quantizedlevel having a value other than zero.

Quantization Matrix: may mean a matrix used in a quantization process ora dequantization process performed to improve subjective or objectiveimage quality. The quantization matrix also may be referred to as ascaling list.

Quantization Matrix Coefficient: may mean each element within aquantization matrix. The quantization matrix coefficient also may bereferred to as a matrix coefficient.

Default Matrix: may mean a predetermined quantization matrixpreliminarily defined in an encoder or a decoder.

Non-default Matrix: may mean a quantization matrix that is notpreliminarily defined in an encoder or a decoder but is signaled by auser.

Statistic Value: a statistic value for at least one among a variable, anencoding parameter, a constant value, etc. which have a computablespecific value may be one or more among an average value, a sum value, aweighted average value, a weighted sum value, the minimum value, themaximum value, the most frequent value, a median value, an interpolatedvalue of the corresponding specific values.

FIG. 1 is a block diagram showing a configuration of an encodingapparatus according to an embodiment to which the present invention isapplied.

An encoding apparatus 100 may be an encoder, a video encoding apparatus,or an image encoding apparatus. A video may include at least one image.The encoding apparatus 100 may sequentially encode at least one image.

Referring to FIG. 1, the encoding apparatus 100 may include a motionprediction unit 111, a motion compensation unit 112, an intra-predictionunit 120, a switch 115, a subtractor 125, a transform unit 130, aquantization unit 140, an entropy encoding unit 150, a dequantizationunit 160, a inverse-transform unit 170, an adder 175, a filter unit 180,and a reference picture buffer 190.

The encoding apparatus 100 may perform encoding of an input image byusing an intra mode or an inter mode or both. In addition, encodingapparatus 100 may generate a bitstream including encoded informationthrough encoding the input image, and output the generated bitstream.The generated bitstream may be stored in a computer readable recordingmedium, or may be streamed through a wired/wireless transmission medium.When an intra mode is used as a prediction mode, the switch 115 may beswitched to an intra. Alternatively, when an inter mode is used as aprediction mode, the switch 115 may be switched to an inter mode.Herein, the intra mode may mean an intra-prediction mode, and the intermode may mean an inter-prediction mode. The encoding apparatus 100 maygenerate a prediction block for an input block of the input image. Inaddition, the encoding apparatus 100 may encode a residual block using aresidual of the input block and the prediction block after theprediction block being generated. The input image may be called as acurrent image that is a current encoding target. The input block may becalled as a current block that is current encoding target, or as anencoding target block.

When a prediction mode is an intra mode, the intra-prediction unit 120may use a sample of a block that has been already encoded/decoded and isadjacent to a current block as a reference sample. The intra-predictionunit 120 may perform spatial prediction for the current block by using areference sample, or generate prediction samples of an input block byperforming spatial prediction. Herein, the intra prediction may meanintra-prediction,

When a prediction mode is an inter mode, the motion prediction unit 111may retrieve a region that best matches with an input block from areference image when performing motion prediction, and deduce a motionvector by using the retrieved region. In this case, a search region maybe used as the region. The reference image may be stored in thereference picture buffer 190. Here, when encoding/decoding for thereference image is performed, it may be stored in the reference picturebuffer 190.

The motion compensation unit 112 may generate a prediction block byperforming motion compensation for the current block using a motionvector. Herein, inter-prediction may mean inter-prediction or motioncompensation.

When the value of the motion vector is not an integer, the motionprediction unit 111 and the motion compensation unit 112 may generatethe prediction block by applying an interpolation filter to a partialregion of the reference picture. In order to perform inter-pictureprediction or motion compensation on a coding unit, it may be determinedthat which mode among a skip mode, a merge mode, an advanced motionvector prediction (AMVP) mode, and a current picture referring mode isused for motion prediction and motion compensation of a prediction unitincluded in the corresponding coding unit. Then, inter-pictureprediction or motion compensation may be differently performed dependingon the determined mode.

The subtractor 125 may generate a residual block by using a differenceof an input block and a prediction block. The residual block may becalled as a residual signal. The residual signal may mean a differencebetween an original signal and a prediction signal. In addition, theresidual signal may be a signal generated by transforming or quantizing,or transforming and quantizing a difference between the original signaland the prediction signal. The residual block may be a residual signalof a block unit.

The transform unit 130 may generate a transform coefficient byperforming transform of a residual block, and output the generatedtransform coefficient. Herein, the transform coefficient may be acoefficient value generated by performing transform of the residualblock. When a transform skip mode is applied, the transform unit 130 mayskip transform of the residual block.

A quantized level may be generated by applying quantization to thetransform coefficient or to the residual signal. Hereinafter, thequantized level may be also called as a transform coefficient inembodiments.

The quantization unit 140 may generate a quantized level by quantizingthe transform coefficient or the residual signal according to aparameter, and output the generated quantized level. Herein, thequantization unit 140 may quantize the transform coefficient by using aquantization matrix.

The entropy encoding unit 150 may generate a bitstream by performingentropy encoding according to a probability distribution on valuescalculated by the quantization unit 140 or on coding parameter valuescalculated when performing encoding, and output the generated bitstream.The entropy encoding unit 150 may perform entropy encoding of sampleinformation of an image and information for decoding an image. Forexample, the information for decoding the image may include a syntaxelement.

When entropy encoding is applied, symbols are represented so that asmaller number of bits are assigned to a symbol having a high chance ofbeing generated and a larger number of bits are assigned to a symbolhaving a low chance of being generated, and thus, the size of bit streamfor symbols to be encoded may be decreased. The entropy encoding unit150 may use an encoding method for entropy encoding such as exponentialGolomb, context-adaptive variable length coding (CAVLC),context-adaptive binary arithmetic coding (CABAC), etc. For example, theentropy encoding unit 150 may perform entropy encoding by using avariable length coding/code (VLC) table. In addition, the entropyencoding unit 150 may deduce a binarization method of a target symboland a probability model of a target symbol/bin, and perform arithmeticcoding by using the deduced binarization method, and a context model.

In order to encode a transform coefficient level (quantized level), theentropy encoding unit 150 may change a two-dimensional block formcoefficient into a one-dimensional vector form by using a transformcoefficient scanning method.

A coding parameter may include information (flag, index, etc.) such assyntax element that is encoded in an encoder and signaled to a decoder,and information derived when performing encoding or decoding. The codingparameter may mean information required when encoding or decoding animage. For example, at least one value or a combination form of aunit/block size, a unit/block depth, unit/block partition information,unit/block shape, unit/block partition structure, whether to partitionof a quad-tree form, whether to partition of a binary-tree form, apartition direction of a binary-tree form (horizontal direction orvertical direction), a partition form of a binary-tree form (symmetricpartition or asymmetric partition), whether or not a current coding unitis partitioned by ternary tree partitioning, direction (horizontal orvertical direction) of the ternary tree partitioning, type (symmetric orasymmetric type) of the ternary tree partitioning, whether a currentcoding unit is partitioned by multi-type tree partitioning, direction(horizontal or vertical direction) of the multi-type three partitioning,type (symmetric or asymmetric type) of the multi-type tree partitioning,and a tree (binary tree or ternary tree) structure of the multi-typetree partitioning, a prediction mode (intra prediction or interprediction), a luma intra-prediction mode/direction, a chromaintra-prediction mode/direction, intra partition information, interpartition information, a coding block partition flag, a prediction blockpartition flag, a transform block partition flag, a reference samplefiltering method, a reference sample filter tab, a reference samplefilter coefficient, a prediction block filtering method, a predictionblock filter tap, a prediction block filter coefficient, a predictionblock boundary filtering method, a prediction block boundary filter tab,a prediction block boundary filter coefficient, an intra-predictionmode, an inter-prediction mode, motion information, a motion vector, amotion vector difference, a reference picture index, a inter-predictionangle, an inter-prediction indicator, a prediction list utilizationflag, a reference picture list, a reference picture, a motion vectorpredictor index, a motion vector predictor candidate, a motion vectorcandidate list, whether to use a merge mode, a merge index, a mergecandidate, a merge candidate list, whether to use a skip mode, aninterpolation filter type, an interpolation filter tab, an interpolationfilter coefficient, a motion vector size, a presentation accuracy of amotion vector, a transform type, a transform size, information ofwhether or not a primary (first) transform is used, information ofwhether or not a secondary transform is used, a primary transform index,a secondary transform index, information of whether or not a residualsignal is present, a coded block pattern, a coded block flag (CBF), aquantization parameter, a quantization parameter residue, a quantizationmatrix, whether to apply an intra loop filter, an intra loop filtercoefficient, an intra loop filter tab, an intra loop filter shape/form,whether to apply a deblocking filter, a deblocking filter coefficient, adeblocking filter tab, a deblocking filter strength, a deblocking filtershape/form, whether to apply an adaptive sample offset, an adaptivesample offset value, an adaptive sample offset category, an adaptivesample offset type, whether to apply an adaptive loop filter, anadaptive loop filter coefficient, an adaptive loop filter tab, anadaptive loop filter shape/form, a binarization/inverse-binarizationmethod, a context model determining method, a context model updatingmethod, whether to perform a regular mode, whether to perform a bypassmode, a context bin, a bypass bin, a significant coefficient flag, alast significant coefficient flag, a coded flag for a unit of acoefficient group, a position of the last significant coefficient, aflag for whether a value of a coefficient is larger than 1, a flag forwhether a value of a coefficient is larger than 2, a flag for whether avalue of a coefficient is larger than 3, information on a remainingcoefficient value, a sign information, a reconstructed luma sample, areconstructed chroma sample, a residual luma sample, a residual chromasample, a luma transform coefficient, a chroma transform coefficient, aquantized luma level, a quantized chroma level, a transform coefficientlevel scanning method, a size of a motion vector search area at adecoder side, a shape of a motion vector search area at a decoder side,a number of time of a motion vector search at a decoder side,information on a CTU size, information on a minimum block size,information on a maximum block size, information on a maximum blockdepth, information on a minimum block depth, an imagedisplaying/outputting sequence, slice identification information, aslice type, slice partition information, tile identificationinformation, a tile type, tile partition information, tile groupidentification information, a tile group type, tile group partitioninformation, a picture type, a bit depth of an input sample, a bit depthof a reconstruction sample, a bit depth of a residual sample, a bitdepth of a transform coefficient, a bit depth of a quantized level, andinformation on a luma signal or information on a chroma signal may beincluded in the coding parameter.

Herein, signaling the flag or index may mean that a corresponding flagor index is entropy encoded and included in a bitstream by an encoder,and may mean that the corresponding flag or index is entropy decodedfrom a bitstream by a decoder.

When the encoding apparatus 100 performs encoding throughinter-prediction, an encoded current image may be used as a referenceimage for another image that is processed afterwards. Accordingly, theencoding apparatus 100 may reconstruct or decode the encoded currentimage, or store the reconstructed or decoded image as a reference imagein reference picture buffer 190.

A quantized level may be dequantized in the dequantization unit 160, ormay be inverse-transformed in the inverse-transform unit 170. Adequantized or inverse-transformed coefficient or both may be added witha prediction block by the adder 175. By adding the dequantized orinverse-transformed coefficient or both with the prediction block, areconstructed block may be generated. Herein, the dequantized orinverse-transformed coefficient or both may mean a coefficient on whichat least one of dequantization and inverse-transform is performed, andmay mean a reconstructed residual block.

A reconstructed block may pass through the filter unit 180. The filterunit 180 may apply at least one of a deblocking filter, a sampleadaptive offset (SAO), and an adaptive loop filter (ALF) to areconstructed sample, a reconstructed block or a reconstructed image.The filter unit 180 may be called as an in-loop filter.

The deblocking filter may remove block distortion generated inboundaries between blocks. In order to determine whether or not to applya deblocking filter, whether or not to apply a deblocking filter to acurrent block may be determined based samples included in several rowsor columns which are included in the block. When a deblocking filter isapplied to a block, another filter may be applied according to arequired deblocking filtering strength.

In order to compensate an encoding error, a proper offset value may beadded to a sample value by using a sample adaptive offset. The sampleadaptive offset may correct an offset of a deblocked image from anoriginal image by a sample unit. A method of partitioning samples of animage into a predetermined number of regions, determining a region towhich an offset is applied, and applying the offset to the determinedregion, or a method of applying an offset in consideration of edgeinformation on each sample may be used.

The adaptive loop filter may perform filtering based on a comparisonresult of the filtered reconstructed image and the original image.Samples included in an image may be partitioned into predeterminedgroups, a filter to be applied to each group may be determined, anddifferential filtering may be performed for each group. Information ofwhether or not to apply the ALF may be signaled by coding units (CUs),and a form and coefficient of the ALF to be applied to each block mayvary.

The reconstructed block or the reconstructed image having passed throughthe filter unit 180 may be stored in the reference picture buffer 190. Areconstructed block processed by the filter unit 180 may be a part of areference image. That is, a reference image is a reconstructed imagecomposed of reconstructed blocks processed by the filter unit 180. Thestored reference image may be used later in inter prediction or motioncompensation.

FIG. 2 is a block diagram showing a configuration of a decodingapparatus according to an embodiment and to which the present inventionis applied.

A decoding apparatus 200 may a decoder, a video decoding apparatus, oran image decoding apparatus.

Referring to FIG. 2, the decoding apparatus 200 may include an entropydecoding unit 210, a dequantization unit 220, a inverse-transform unit230, an intra-prediction unit 240, a motion compensation unit 250, anadder 225, a filter unit 260, and a reference picture buffer 270.

The decoding apparatus 200 may receive a bitstream output from theencoding apparatus 100. The decoding apparatus 200 may receive abitstream stored in a computer readable recording medium, or may receivea bitstream that is streamed through a wired/wireless transmissionmedium. The decoding apparatus 200 may decode the bitstream by using anintra mode or an inter mode. In addition, the decoding apparatus 200 maygenerate a reconstructed image generated through decoding or a decodedimage, and output the reconstructed image or decoded image.

When a prediction mode used when decoding is an intra mode, a switch maybe switched to an intra. Alternatively, when a prediction mode used whendecoding is an inter mode, a switch may be switched to an inter mode.

The decoding apparatus 200 may obtain a reconstructed residual block bydecoding the input bitstream, and generate a prediction block. When thereconstructed residual block and the prediction block are obtained, thedecoding apparatus 200 may generate a reconstructed block that becomes adecoding target by adding the reconstructed residual block with theprediction block. The decoding target block may be called a currentblock.

The entropy decoding unit 210 may generate symbols by entropy decodingthe bitstream according to a probability distribution. The generatedsymbols may include a symbol of a quantized level form. Herein, anentropy decoding method may be a inverse-process of the entropy encodingmethod described above.

In order to decode a transform coefficient level (quantized level), theentropy decoding unit 210 may change a one-directional vector formcoefficient into a two-dimensional block form by using a transformcoefficient scanning method.

A quantized level may be dequantized in the dequantization unit 220, orinverse-transformed in the inverse-transform unit 230. The quantizedlevel may be a result of dequantizing or inverse-transforming or both,and may be generated as a reconstructed residual block. Herein, thedequantization unit 220 may apply a quantization matrix to the quantizedlevel.

When an intra mode is used, the intra-prediction unit 240 may generate aprediction block by performing, for the current block, spatialprediction that uses a sample value of a block adjacent to a decodingtarget block and which has been already decoded.

When an inter mode is used, the motion compensation unit 250 maygenerate a prediction block by performing, for the current block, motioncompensation that uses a motion vector and a reference image stored inthe reference picture buffer 270.

The adder 225 may generate a reconstructed block by adding thereconstructed residual block with the prediction block. The filter unit260 may apply at least one of a deblocking filter, a sample adaptiveoffset, and an adaptive loop filter to the reconstructed block orreconstructed image. The filter unit 260 may output the reconstructedimage. The reconstructed block or reconstructed image may be stored inthe reference picture buffer 270 and used when performinginter-prediction. A reconstructed block processed by the filter unit 260may be a part of a reference image. That is, a reference image is areconstructed image composed of reconstructed blocks processed by thefilter unit 260. The stored reference image may be used later in interprediction or motion compensation.

FIG. 3 is a view schematically showing a partition structure of an imagewhen encoding and decoding the image. FIG. 3 schematically shows anexample of partitioning a single unit into a plurality of lower units.

In order to efficiently partition an image, when encoding and decoding,a coding unit (CU) may be used. The coding unit may be used as a basicunit when encoding/decoding the image. In addition, the coding unit maybe used as a unit for distinguishing an intra prediction mode and aninter prediction mode when encoding/decoding the image. The coding unitmay be a basic unit used for prediction, transform, quantization,inverse-transform, dequantization, or an encoding/decoding process of atransform coefficient.

Referring to FIG. 3, an image 300 is sequentially partitioned in alargest coding unit (LCU), and a LCU unit is determined as a partitionstructure. Herein, the LCU may be used in the same meaning as a codingtree unit (CTU). A unit partitioning may mean partitioning a blockassociated with to the unit. In block partition information, informationof a unit depth may be included. Depth information may represent anumber of times or a degree or both in which a unit is partitioned. Asingle unit may be partitioned into a plurality of lower level unitshierarchically associated with depth information based on a treestructure. In other words, a unit and a lower level unit generated bypartitioning the unit may correspond to a node and a child node of thenode, respectively. Each of partitioned lower unit may have depthinformation. Depth information may be information representing a size ofa CU, and may be stored in each CU. Unit depth represents times and/ordegrees related to partitioning a unit. Therefore, partitioninginformation of a lower-level unit may comprise information on a size ofthe lower-level unit.

A partition structure may mean a distribution of a coding unit (CU)within an LCU 310. Such a distribution may be determined according towhether or not to partition a single CU into a plurality (positiveinteger equal to or greater than 2 including 2, 4, 8, 16, etc.) of CUs.A horizontal size and a vertical size of the CU generated bypartitioning may respectively be half of a horizontal size and avertical size of the CU before partitioning, or may respectively havesizes smaller than a horizontal size and a vertical size beforepartitioning according to a number of times of partitioning. The CU maybe recursively partitioned into a plurality of CUs. By the recursivepartitioning, at least one among a height and a width of a CU afterpartitioning may decrease comparing with at least one among a height anda width of a CU before partitioning. Partitioning of the CU may berecursively performed until to a predefined depth or predefined size.For example, a depth of an LCU may be 0, and a depth of a smallestcoding unit (SCU) may be a predefined maximum depth. Herein, the LCU maybe a coding unit having a maximum coding unit size, and the SCU may be acoding unit having a minimum coding unit size as described above.Partitioning is started from the LCU 310, a CU depth increases by 1 as ahorizontal size or a vertical size or both of the CU decreases bypartitioning. For example, for each depth, a CU which is not partitionedmay have a size of 2N×2N. Also, in case of a CU which is partitioned, aCU with a size of 2N×2N may be partitioned into four CUs with a size ofN×N. A size of N may decrease to half as a depth increase by 1.

In addition, information whether or not the CU is partitioned may berepresented by using partition information of the CU. The partitioninformation may be 1-bit information. All CUs, except for a SCU, mayinclude partition information. For example, when a value of partitioninformation is a first value, the CU may not be partitioned, when avalue of partition information is a second value, the CU may bepartitioned

Referring to FIG. 3, an LCU having a depth 0 may be a 64×64 block. 0 maybe a minimum depth. A SCU having a depth 3 may be an 8×8 block. 3 may bea maximum depth. A CU of a 32×32 block and a 16×16 block may berespectively represented as a depth 1 and a depth 2.

For example, when a single coding unit is partitioned into four codingunits, a horizontal size and a vertical size of the four partitionedcoding units may be a half size of a horizontal and vertical size of theCU before being partitioned. In one embodiment, when a coding unithaving a 32×32 size is partitioned into four coding units, each of thefour partitioned coding units may have a 16×16 size. When a singlecoding unit is partitioned into four coding units, it may be called thatthe coding unit may be partitioned into a quad-tree form.

For example, when one coding unit is partitioned into two sub-codingunits, the horizontal or vertical size (width or height) of each of thetwo sub-coding units may be half the horizontal or vertical size of theoriginal coding unit. For example, when a coding unit having a size of32×32 is vertically partitioned into two sub-coding units, each of thetwo sub-coding units may have a size of 16×32. For example, when acoding unit having a size of 8×32 is horizontally partitioned into twosub-coding units, each of the two sub-coding units may have a size of8×16. When one coding unit is partitioned into two sub-coding units, itcan be said that the coding unit is binary-partitioned or is partitionedby a binary tree partition structure.

For example, when one coding unit is partitioned into three sub-codingunits, the horizontal or vertical size of the coding unit can bepartitioned with a ratio of 1:2:1, thereby producing three sub-codingunits whose horizontal or vertical sizes are in a ratio of 1:2:1. Forexample, when a coding unit having a size of 16×32 is horizontallypartitioned into three sub-coding units, the three sub-coding units mayhave sizes of 16×8, 16×16, and 16×8 respectively, in the order from theuppermost to the lowermost sub-coding unit. For example, when a codingunit having a size of 32×32 is vertically split into three sub-codingunits, the three sub-coding units may have sizes of 8×32, 16×32, and8×32, respectively in the order from the left to the right sub-codingunit. When one coding unit is partitioned into three sub-coding units,it can be said that the coding unit is ternary-partitioned orpartitioned by a ternary tree partition structure.

In FIG. 3, a coding tree unit (CTU) 320 is an example of a CTU to whicha quad tree partition structure, a binary tree partition structure, anda ternary tree partition structure are all applied.

As described above, in order to partition the CTU, at least one of aquad tree partition structure, a binary tree partition structure, and aternary tree partition structure may be applied. Various tree partitionstructures may be sequentially applied to the CTU, according to apredetermined priority order. For example, the quad tree partitionstructure may be preferentially applied to the CTU. A coding unit thatcannot be partitioned any longer using a quad tree partition structuremay correspond to a leaf node of a quad tree. A coding unitcorresponding to a leaf node of a quad tree may serve as a root node ofa binary and/or ternary tree partition structure. That is, a coding unitcorresponding to a leaf node of a quad tree may be further partitionedby a binary tree partition structure or a ternary tree partitionstructure, or may not be further partitioned. Therefore, by preventing acoding block that results from binary tree partitioning or ternary treepartitioning of a coding unit corresponding to a leaf node of a quadtree from undergoing further quad tree partitioning, block partitioningand/or signaling of partition information can be effectively performed.

The fact that a coding unit corresponding to a node of a quad tree ispartitioned may be signaled using quad partition information. The quadpartition information having a first value (e.g., “1”) may indicate thata current coding unit is partitioned by the quad tree partitionstructure. The quad partition information having a second value (e.g.,“0”) may indicate that a current coding unit is not partitioned by thequad tree partition structure. The quad partition information may be aflag having a predetermined length (e.g., one bit).

There may not be a priority between the binary tree partitioning and theternary tree partitioning. That is, a coding unit corresponding to aleaf node of a quad tree may further undergo arbitrary partitioningamong the binary tree partitioning and the ternary tree partitioning. Inaddition, a coding unit generated through the binary tree partitioningor the ternary tree partitioning may undergo a further binary treepartitioning or a further ternary tree partitioning, or may not befurther partitioned.

A tree structure in which there is no priority among the binary treepartitioning and the ternary tree partitioning is referred to as amulti-type tree structure. A coding unit corresponding to a leaf node ofa quad tree may serve as a root node of a multi-type tree. Whether topartition a coding unit which corresponds to a node of a multi-type treemay be signaled using at least one of multi-type tree partitionindication information, partition direction information, and partitiontree information. For partitioning of a coding unit corresponding to anode of a multi-type tree, the multi-type tree partition indicationinformation, the partition direction, and the partition tree informationmay be sequentially signaled.

The multi-type tree partition indication information having a firstvalue (e.g., “1”) may indicate that a current coding unit is to undergoa multi-type tree partitioning. The multi-type tree partition indicationinformation having a second value (e.g., “0”) may indicate that acurrent coding unit is not to undergo a multi-type tree partitioning.

When a coding unit corresponding to a node of a multi-type tree isfurther partitioned by a multi-type tree partition structure, the codingunit may include partition direction information. The partitiondirection information may indicate in which direction a current codingunit is to be partitioned for the multi-type tree partitioning. Thepartition direction information having a first value (e.g., “1”) mayindicate that a current coding unit is to be vertically partitioned. Thepartition direction information having a second value (e.g., “0”) mayindicate that a current coding unit is to be horizontally partitioned.

When a coding unit corresponding to a node of a multi-type tree isfurther partitioned by a multi-type tree partition structure, thecurrent coding unit may include partition tree information. Thepartition tree information may indicate a tree partition structure whichis to be used for partitioning of a node of a multi-type tree. Thepartition tree information having a first value (e.g., “1”) may indicatethat a current coding unit is to be partitioned by a binary treepartition structure. The partition tree information having a secondvalue (e.g., “0”) may indicate that a current coding unit is to bepartitioned by a ternary tree partition structure.

The partition indication information, the partition tree information,and the partition direction information may each be a flag having apredetermined length (e.g., one bit).

At least any one of the quadtree partition indication information, themulti-type tree partition indication information, the partitiondirection information, and the partition tree information may be entropyencoded/decoded. For the entropy-encoding/decoding of those types ofinformation, information on a neighboring coding unit adjacent to thecurrent coding unit may be used. For example, there is a highprobability that the partition type (the partitioned or non-partitioned,the partition tree, and/or the partition direction) of a leftneighboring coding unit and/or an upper neighboring coding unit of acurrent coding unit is similar to that of the current coding unit.Therefore, context information for entropy encoding/decoding of theinformation on the current coding unit may be derived from theinformation on the neighboring coding units. The information on theneighboring coding units may include at least any one of quad partitioninformation, multi-type tree partition indication information, partitiondirection information, and partition tree information.

As another example, among binary tree partitioning and ternary treepartitioning, binary tree partitioning may be preferentially performed.That is, a current coding unit may primarily undergo binary treepartitioning, and then a coding unit corresponding to a leaf node of abinary tree may be set as a root node for ternary tree partitioning. Inthis case, neither quad tree partitioning nor binary tree partitioningmay not be performed on the coding unit corresponding to a node of aternary tree.

A coding unit that cannot be partitioned by a quad tree partitionstructure, a binary tree partition structure, and/or a ternary treepartition structure becomes a basic unit for coding, prediction and/ortransformation. That is, the coding unit cannot be further partitionedfor prediction and/or transformation. Therefore, the partition structureinformation and the partition information used for partitioning a codingunit into prediction units and/or transformation units may not bepresent in a bit stream.

However, when the size of a coding unit (i.e., a basic unit forpartitioning) is larger than the size of a maximum transformation block,the coding unit may be recursively partitioned until the size of thecoding unit is reduced to be equal to or smaller than the size of themaximum transformation block. For example, when the size of a codingunit is 64×64 and when the size of a maximum transformation block is32×32, the coding unit may be partitioned into four 32×32 blocks fortransformation. For example, when the size of a coding unit is 32×64 andthe size of a maximum transformation block is 32×32, the coding unit maybe partitioned into two 32×32 blocks for the transformation. In thiscase, the partitioning of the coding unit for transformation is notsignaled separately, and may be determined through comparison betweenthe horizontal or vertical size of the coding unit and the horizontal orvertical size of the maximum transformation block. For example, when thehorizontal size (width) of the coding unit is larger than the horizontalsize (width) of the maximum transformation block, the coding unit may bevertically bisected. For example, when the vertical size (length) of thecoding unit is larger than the vertical size (length) of the maximumtransformation block, the coding unit may be horizontally bisected.

Information of the maximum and/or minimum size of the coding unit andinformation of the maximum and/or minimum size of the transformationblock may be signaled or determined at an upper level of the codingunit. The upper level may be, for example, a sequence level, a picturelevel, a slice level, a tile group level, a tile level, or the like. Forexample, the minimum size of the coding unit may be determined to be4×4. For example, the maximum size of the transformation block may bedetermined to be 64×64. For example, the minimum size of thetransformation block may be determined to be 4×4.

Information of the minimum size (quad tree minimum size) of a codingunit corresponding to a leaf node of a quad tree and/or information ofthe maximum depth (the maximum tree depth of a multi-type tree) from aroot node to a leaf node of the multi-type tree may be signaled ordetermined at an upper level of the coding unit. For example, the upperlevel may be a sequence level, a picture level, a slice level, a tilegroup level, a tile level, or the like. Information of the minimum sizeof a quad tree and/or information of the maximum depth of a multi-typetree may be signaled or determined for each of an intra-picture sliceand an inter-picture slice.

Difference information between the size of a CTU and the maximum size ofa transformation block may be signaled or determined at an upper levelof the coding unit. For example, the upper level may be a sequencelevel, a picture level, a slice level, a tile group level, a tile level,or the like. Information of the maximum size of the coding unitscorresponding to the respective nodes of a binary tree (hereinafter,referred to as a maximum size of a binary tree) may be determined basedon the size of the coding tree unit and the difference information. Themaximum size of the coding units corresponding to the respective nodesof a ternary tree (hereinafter, referred to as a maximum size of aternary tree) may vary depending on the type of slice. For example, foran intra-picture slice, the maximum size of a ternary tree may be 32×32.For example, for an inter-picture slice, the maximum size of a ternarytree may be 128×128. For example, the minimum size of the coding unitscorresponding to the respective nodes of a binary tree (hereinafter,referred to as a minimum size of a binary tree) and/or the minimum sizeof the coding units corresponding to the respective nodes of a ternarytree (hereinafter, referred to as a minimum size of a ternary tree) maybe set as the minimum size of a coding block.

As another example, the maximum size of a binary tree and/or the maximumsize of a ternary tree may be signaled or determined at the slice level.Alternatively, the minimum size of the binary tree and/or the minimumsize of the ternary tree may be signaled or determined at the slicelevel.

Depending on size and depth information of the above-described variousblocks, quad partition information, multi-type tree partition indicationinformation, partition tree information and/or partition directioninformation may be included or may not be included in a bit stream.

For example, when the size of the coding unit is not larger than theminimum size of a quad tree, the coding unit does not contain quadpartition information. Thus, the quad partition information may bededuced from a second value.

For example, when the sizes (horizontal and vertical sizes) of a codingunit corresponding to a node of a multi-type tree are larger than themaximum sizes (horizontal and vertical sizes) of a binary tree and/orthe maximum sizes (horizontal and vertical sizes) of a ternary tree, thecoding unit may not be binary-partitioned or ternary-partitioned.Accordingly, the multi-type tree partition indication information maynot be signaled but may be deduced from a second value.

Alternatively, when the sizes (horizontal and vertical sizes) of acoding unit corresponding to a node of a multi-type tree are the same asthe maximum sizes (horizontal and vertical sizes) of a binary treeand/or are two times as large as the maximum sizes (horizontal andvertical sizes) of a ternary tree, the coding unit may not be furtherbinary-partitioned or ternary-partitioned. Accordingly, the multi-typetree partition indication information may not be signaled but be derivedfrom a second value. This is because when a coding unit is partitionedby a binary tree partition structure and/or a ternary tree partitionstructure, a coding unit smaller than the minimum size of a binary treeand/or the minimum size of a ternary tree is generated.

Alternatively, the binary tree partitioning or the ternary treepartitioning may be limited on the basis of the size of a virtualpipeline data unit (hereinafter, a pipeline buffer size). For example,when the coding unit is divided into sub-coding units which do not fitthe pipeline buffer size by the binary tree partitioning or the ternarytree partitioning, the corresponding binary tree partitioning or ternarytree partitioning may be limited. The pipeline buffer size may be thesize of the maximum transform block (e.g., 64×64). For example, when thepipeline buffer size is 64×64, the division below may be limited.

-   -   N×M (N and/or M is 128) Ternary tree partitioning for coding        units    -   128×N (N<=64) Binary tree partitioning in horizontal direction        for coding units    -   N×128 (N<=64) Binary tree partitioning in vertical direction for        coding units

Alternatively, when the depth of a coding unit corresponding to a nodeof a multi-type tree is equal to the maximum depth of the multi-typetree, the coding unit may not be further binary-partitioned and/orternary-partitioned. Accordingly, the multi-type tree partitionindication information may not be signaled but may be deduced from asecond value.

Alternatively, only when at least one of vertical direction binary treepartitioning, horizontal direction binary tree partitioning, verticaldirection ternary tree partitioning, and horizontal direction ternarytree partitioning is possible for a coding unit corresponding to a nodeof a multi-type tree, the multi-type tree partition indicationinformation may be signaled. Otherwise, the coding unit may not bebinary-partitioned and/or ternary-partitioned. Accordingly, themulti-type tree partition indication information may not be signaled butmay be deduced from a second value.

Alternatively, only when both of the vertical direction binary treepartitioning and the horizontal direction binary tree partitioning orboth of the vertical direction ternary tree partitioning and thehorizontal direction ternary tree partitioning are possible for a codingunit corresponding to a node of a multi-type tree, the partitiondirection information may be signaled. Otherwise, the partitiondirection information may not be signaled but may be derived from avalue indicating possible partitioning directions.

Alternatively, only when both of the vertical direction binary treepartitioning and the vertical direction ternary tree partitioning orboth of the horizontal direction binary tree partitioning and thehorizontal direction ternary tree partitioning are possible for a codingtree corresponding to a node of a multi-type tree, the partition treeinformation may be signaled. Otherwise, the partition tree informationmay not be signaled but be deduced from a value indicating a possiblepartitioning tree structure.

FIG. 4 is a view showing an intra-prediction process.

Arrows from center to outside in FIG. 4 may represent predictiondirections of intra prediction modes.

Intra encoding and/or decoding may be performed by using a referencesample of a neighbor block of the current block. A neighbor block may bea reconstructed neighbor block. For example, intra encoding and/ordecoding may be performed by using an encoding parameter or a value of areference sample included in a reconstructed neighbor block.

A prediction block may mean a block generated by performing intraprediction. A prediction block may correspond to at least one among CU,PU and TU. A unit of a prediction block may have a size of one among CU,PU and TU. A prediction block may be a square block having a size of2×2, 4×4, 16×16, 32×32 or 64×64 etc. or may be a rectangular blockhaving a size of 2×8, 4×8, 2×16, 4×16 and 8×16 etc.

Intra prediction may be performed according to intra prediction mode forthe current block. The number of intra prediction modes which thecurrent block may have may be a fixed value and may be a valuedetermined differently according to an attribute of a prediction block.For example, an attribute of a prediction block may comprise a size of aprediction block and a shape of a prediction block, etc.

The number of intra-prediction modes may be fixed to N regardless of ablock size. Or, the number of intra prediction modes may be 3, 5, 9, 17,34, 35, 36, 65, or 67 etc. Alternatively, the number of intra-predictionmodes may vary according to a block size or a color component type orboth. For example, the number of intra prediction modes may varyaccording to whether the color component is a luma signal or a chromasignal. For example, as a block size becomes large, a number ofintra-prediction modes may increase. Alternatively, a number ofintra-prediction modes of a luma component block may be larger than anumber of intra-prediction modes of a chroma component block.

An intra-prediction mode may be a non-angular mode or an angular mode.The non-angular mode may be a DC mode or a planar mode, and the angularmode may be a prediction mode having a specific direction or angle. Theintra-prediction mode may be expressed by at least one of a mode number,a mode value, a mode numeral, a mode angle, and mode direction. A numberof intra-prediction modes may be M, which is larger than 1, includingthe non-angular and the angular mode. In order to intra-predict acurrent block, a step of determining whether or not samples included ina reconstructed neighbor block may be used as reference samples of thecurrent block may be performed. When a sample that is not usable as areference sample of the current block is present, a value obtained byduplicating or performing interpolation on at least one sample valueamong samples included in the reconstructed neighbor block or both maybe used to replace with a non-usable sample value of a sample, thus thereplaced sample value is used as a reference sample of the currentblock.

FIG. 7 is a diagram illustrating reference samples capable of being usedfor intra prediction.

As shown in FIG. 7, at least one of the reference sample line 0 to thereference sample line 3 may be used for intra prediction of the currentblock. In FIG. 7, the samples of a segment A and a segment F may bepadded with the samples closest to a segment B and a segment E,respectively, instead of retrieving from the reconstructed neighboringblock. Index information indicating the reference sample line to be usedfor intra prediction of the current block may be signaled. When theupper boundary of the current block is the boundary of the CTU, only thereference sample line 0 may be available. Therefore, in this case, theindex information may not be signaled. When a reference sample lineother than the reference sample line 0 is used, filtering for aprediction block, which will be described later, may not be performed.

When intra-predicting, a filter may be applied to at least one of areference sample and a prediction sample based on an intra-predictionmode and a current block size.

In case of a planar mode, when generating a prediction block of acurrent block, according to a position of a prediction target samplewithin a prediction block, a sample value of the prediction targetsample may be generated by using a weighted sum of an upper and leftside reference sample of a current sample, and a right upper side andleft lower side reference sample of the current block. In addition, incase of a DC mode, when generating a prediction block of a currentblock, an average value of upper side and left side reference samples ofthe current block may be used. In addition, in case of an angular mode,a prediction block may be generated by using an upper side, a left side,a right upper side, and/or a left lower side reference sample of thecurrent block. In order to generate a prediction sample value,interpolation of a real number unit may be performed.

In the case of intra prediction between color components, a predictionblock for the current block of the second color component may begenerated on the basis of the corresponding reconstructed block of thefirst color component. For example, the first color component may be aluma component, and the second color component may be a chromacomponent. For intra prediction between color components, the parametersof the linear model between the first color component and the secondcolor component may be derived on the basis of the template. Thetemplate may include upper and/or left neighboring samples of thecurrent block and upper and/or left neighboring samples of thereconstructed block of the first color component corresponding thereto.For example, the parameters of the linear model may be derived using asample value of a first color component having a maximum value amongsamples in a template and a sample value of a second color componentcorresponding thereto, and a sample value of a first color componenthaving a minimum value among samples in the template and a sample valueof a second color component corresponding thereto. When the parametersof the linear model are derived, a corresponding reconstructed block maybe applied to the linear model to generate a prediction block for thecurrent block. According to a video format, subsampling may be performedon the neighboring samples of the reconstructed block of the first colorcomponent and the corresponding reconstructed block. For example, whenone sample of the second color component corresponds to four samples ofthe first color component, four samples of the first color component maybe sub-sampled to compute one corresponding sample. In this case, theparameter derivation of the linear model and intra prediction betweencolor components may be performed on the basis of the correspondingsub-sampled samples. Whether or not to perform intra prediction betweencolor components and/or the range of the template may be signaled as theintra prediction mode.

The current block may be partitioned into two or four sub-blocks in thehorizontal or vertical direction. The partitioned sub-blocks may besequentially reconstructed. That is, the intra prediction may beperformed on the sub-block to generate the sub-prediction block. Inaddition, dequantization and/or inverse transform may be performed onthe sub-blocks to generate sub-residual blocks. A reconstructedsub-block may be generated by adding the sub-prediction block to thesub-residual block. The reconstructed sub-block may be used as areference sample for intra prediction of the sub-sub-blocks. Thesub-block may be a block including a predetermined number (for example,16) or more samples. Accordingly, for example, when the current block isan 8×4 block or a 4×8 block, the current block may be partitioned intotwo sub-blocks. Also, when the current block is a 4×4 block, the currentblock may not be partitioned into sub-blocks. When the current block hasother sizes, the current block may be partitioned into four sub-blocks.Information on whether or not to perform the intra prediction based onthe sub-blocks and/or the partitioning direction (horizontal orvertical) may be signaled. The intra prediction based on the sub-blocksmay be limited to be performed only when reference sample line 0 isused. When the intra prediction based on the sub-block is performed,filtering for the prediction block, which will be described later, maynot be performed.

The final prediction block may be generated by performing filtering onthe prediction block that is intra-predicted. The filtering may beperformed by applying predetermined weights to the filtering targetsample, the left reference sample, the upper reference sample, and/orthe upper left reference sample. The weight and/or the reference sample(range, position, etc.) used for the filtering may be determined on thebasis of at least one of a block size, an intra prediction mode, and aposition of the filtering target sample in the prediction block. Thefiltering may be performed only in the case of a predetermined intraprediction mode (e.g., DC, planar, vertical, horizontal, diagonal,and/or adjacent diagonal modes). The adjacent diagonal mode may be amode in which k is added to or subtracted from the diagonal mode. Forexample, k may be a positive integer of 8 or less.

An intra-prediction mode of a current block may be entropyencoded/decoded by predicting an intra-prediction mode of a blockpresent adjacent to the current block. When intra-prediction modes ofthe current block and the neighbor block are identical, information thatthe intra-prediction modes of the current block and the neighbor blockare identical may be signaled by using predetermined flag information.In addition, indicator information of an intra-prediction mode that isidentical to the intra-prediction mode of the current block amongintra-prediction modes of a plurality of neighbor blocks may besignaled. When intra-prediction modes of the current block and theneighbor block are different, intra-prediction mode information of thecurrent block may be entropy encoded/decoded by performing entropyencoding/decoding based on the intra-prediction mode of the neighborblock.

FIG. 5 is a diagram illustrating an embodiment of an inter-pictureprediction process.

In FIG. 5, a rectangle may represent a picture. In FIG. 5, an arrowrepresents a prediction direction. Pictures may be categorized intointra pictures (I pictures), predictive pictures (P pictures), andBi-predictive pictures (B pictures) according to the encoding typethereof.

The I picture may be encoded through intra-prediction without requiringinter-picture prediction. The P picture may be encoded throughinter-picture prediction by using a reference picture that is present inone direction (i.e., forward direction or backward direction) withrespect to a current block. The B picture may be encoded throughinter-picture prediction by using reference pictures that are preset intwo directions (i.e., forward direction and backward direction) withrespect to a current block. When the inter-picture prediction is used,the encoder may perform inter-picture prediction or motion compensationand the decoder may perform the corresponding motion compensation.

Hereinbelow, an embodiment of the inter-picture prediction will bedescribed in detail.

The inter-picture prediction or motion compensation may be performedusing a reference picture and motion information.

Motion information of a current block may be derived duringinter-picture prediction by each of the encoding apparatus 100 and thedecoding apparatus 200. The motion information of the current block maybe derived by using motion information of a reconstructed neighboringblock, motion information of a collocated block (also referred to as acol block or a co-located block), and/or a block adjacent to theco-located block. The co-located block may mean a block that is locatedspatially at the same position as the current block, within a previouslyreconstructed collocated picture (also referred to as a col picture or aco-located picture). The co-located picture may be one picture among oneor more reference pictures included in a reference picture list.

The derivation method of the motion information may be differentdepending on the prediction mode of the current block. For example, aprediction mode applied for inter prediction includes an AMVP mode, amerge mode, a skip mode, a merge mode with a motion vector difference, asubblock merge mode, a triangle partition mode, an inter-intracombination prediction mode, affine mode, and the like. Herein, themerge mode may be referred to as a motion merge mode.

For example, when the AMVP is used as the prediction mode, at least oneof motion vectors of the reconstructed neighboring blocks, motionvectors of the co-located blocks, motion vectors of blocks adjacent tothe co-located blocks, and a (0, 0) motion vector may be determined asmotion vector candidates for the current block, and a motion vectorcandidate list is generated by using the emotion vector candidates. Themotion vector candidate of the current block can be derived by using thegenerated motion vector candidate list. The motion information of thecurrent block may be determined based on the derived motion vectorcandidate. The motion vectors of the collocated blocks or the motionvectors of the blocks adjacent to the collocated blocks may be referredto as temporal motion vector candidates, and the motion vectors of thereconstructed neighboring blocks may be referred to as spatial motionvector candidates.

The encoding apparatus 100 may calculate a motion vector difference(MVD) between the motion vector of the current block and the motionvector candidate and may perform entropy encoding on the motion vectordifference (MVD). In addition, the encoding apparatus 100 may performentropy encoding on a motion vector candidate index and generate abitstream. The motion vector candidate index may indicate an optimummotion vector candidate among the motion vector candidates included inthe motion vector candidate list. The decoding apparatus may performentropy decoding on the motion vector candidate index included in thebitstream and may select a motion vector candidate of a decoding targetblock from among the motion vector candidates included in the motionvector candidate list by using the entropy-decoded motion vectorcandidate index. In addition, the decoding apparatus 200 may add theentropy-decoded MVD and the motion vector candidate extracted throughthe entropy decoding, thereby deriving the motion vector of the decodingtarget block.

Meanwhile, the coding apparatus 100 may perform entropy-coding onresolution information of the calculated MVD. The decoding apparatus 200may adjust the resolution of the entropy-decoded MVD using the MVDresolution information.

Meanwhile, the coding apparatus 100 calculates a motion vectordifference (MVD) between a motion vector and a motion vector candidatein the current block on the basis of an affine model, and performsentropy-coding on the MVD. The decoding apparatus 200 derives a motionvector on a per sub-block basis by deriving an affine control motionvector of a decoding target block through the sum of the entropy-decodedMVD and an affine control motion vector candidate.

The bitstream may include a reference picture index indicating areference picture. The reference picture index may be entropy-encoded bythe encoding apparatus 100 and then signaled as a bitstream to thedecoding apparatus 200. The decoding apparatus 200 may generate aprediction block of the decoding target block based on the derivedmotion vector and the reference picture index information.

Another example of the method of deriving the motion information of thecurrent may be the merge mode. The merge mode may mean a method ofmerging motion of a plurality of blocks. The merge mode may mean a modeof deriving the motion information of the current block from the motioninformation of the neighboring blocks. When the merge mode is applied,the merge candidate list may be generated using the motion informationof the reconstructed neighboring blocks and/or the motion information ofthe collocated blocks. The motion information may include at least oneof a motion vector, a reference picture index, and an inter-pictureprediction indicator. The prediction indicator may indicateone-direction prediction (L0 prediction or L1 prediction) ortwo-direction predictions (L0 prediction and L1 prediction).

The merge candidate list may be a list of motion information stored. Themotion information included in the merge candidate list may be at leastone of motion information (spatial merge candidate) of a neighboringblock adjacent to the current block, motion information (temporal mergecandidate) of the collocated block of the current block in the referencepicture, new motion information generated by a combination of the motioninformation exiting in the merge candidate list, motion information(history-based merge candidate) of the block that is encoded/decodedbefore the current block, and zero merge candidate.

The encoding apparatus 100 may generate a bitstream by performingentropy encoding on at least one of a merge flag and a merge index andmay signal the bitstream to the decoding apparatus 200. The merge flagmay be information indicating whether or not to perform the merge modefor each block, and the merge index may be information indicating thatwhich neighboring block, among the neighboring blocks of the currentblock, is a merge target block. For example, the neighboring blocks ofthe current block may include a left neighboring block on the left sideof the current block, an upper neighboring block disposed above thecurrent block, and a temporal neighboring block temporally adjacent tothe current block.

Meanwhile, the coding apparatus 100 performs entropy-coding on thecorrection information for correcting the motion vector among the motioninformation of the merge candidate and signals the same to the decodingapparatus 200. The decoding apparatus 200 can correct the motion vectorof the merge candidate selected by the merge index on the basis of thecorrection information. Here, the correction information may include atleast one of information on whether or not to perform the correction,correction direction information, and correction size information. Asdescribed above, the prediction mode that corrects the motion vector ofthe merge candidate on the basis of the signaled correction informationmay be referred to as a merge mode having the motion vector difference.

The skip mode may be a mode in which the motion information of theneighboring block is applied to the current block as it is. When theskip mode is applied, the encoding apparatus 100 may perform entropyencoding on information of the fact that the motion information of whichblock is to be used as the motion information of the current block togenerate a bit stream, and may signal the bitstream to the decodingapparatus 200. The encoding apparatus 100 may not signal a syntaxelement regarding at least any one of the motion vector differenceinformation, the encoding block flag, and the transform coefficientlevel to the decoding apparatus 200.

The subblock merge mode may mean a mode that derives the motioninformation in units of sub-blocks of a coding block (CU). When thesubblock merge mode is applied, a subblock merge candidate list may begenerated using motion information (sub-block based temporal mergecandidate) of the sub-block collocated to the current sub-block in thereference image and/or an affine control point motion vector mergecandidate.

The triangle partition mode may mean a mode that derives motioninformation by partitioning the current block into diagonal directions,derives each prediction sample using each of the derived motioninformation, and derives the prediction sample of the current block byweighting each of the derived prediction samples.

The inter-intra combined prediction mode may mean a mode that derives aprediction sample of the current block by weighting a prediction samplegenerated by inter prediction and a prediction sample generated by intraprediction.

The decoding apparatus 200 may correct the derived motion information byitself. The decoding apparatus 200 may search the predetermined regionon the basis of the reference block indicated by the derived motioninformation and derive the motion information having the minimum SAD asthe corrected motion information.

The decoding apparatus 200 may compensate a prediction sample derivedvia inter prediction using an optical flow.

FIG. 6 is a diagram illustrating a transform and quantization process.

As illustrated in FIG. 6, a transform and/or quantization process isperformed on a residual signal to generate a quantized level signal. Theresidual signal is a difference between an original block and aprediction block (i.e., an intra prediction block or an inter predictionblock). The prediction block is a block generated through intraprediction or inter prediction. The transform may be a primarytransform, a secondary transform, or both. The primary transform of theresidual signal results in transform coefficients, and the secondarytransform of the transform coefficients results in secondary transformcoefficients.

At least one scheme selected from among various transform schemes whichare preliminarily defined is used to perform the primary transform. Forexample, examples of the predefined transform schemes include discretecosine transform (DCT), discrete sine transform (DST), andKarhunen-Loeve transform (KLT). The transform coefficients generatedthrough the primary transform may undergo the secondary transform. Thetransform schemes used for the primary transform and/or the secondarytransform may be determined according to coding parameters of thecurrent block and/or neighboring blocks of the current block.Alternatively, transform information indicating the transform scheme maybe signaled. The DCT-based transform may include, for example, DCT-2,DCT-8, and the like. The DST-based transform may include, for example,DST-7.

A quantized-level signal (quantization coefficients) may be generated byperforming quantization on the residual signal or a result of performingthe primary transform and/or the secondary transform. The quantizedlevel signal may be scanned according to at least one of a diagonalup-right scan, a vertical scan, and a horizontal scan, depending on anintra prediction mode of a block or a block size/shape. For example, asthe coefficients are scanned in a diagonal up-right scan, thecoefficients in a block form change into a one-dimensional vector form.Aside from the diagonal up-right scan, the horizontal scan ofhorizontally scanning a two-dimensional block form of coefficients orthe vertical scan of vertically scanning a two-dimensional block form ofcoefficients may be used depending on the intra prediction mode and/orthe size of a transform block. The scanned quantized-level coefficientsmay be entropy-encoded to be inserted into a bitstream.

A decoder entropy-decodes the bitstream to obtain the quantized-levelcoefficients. The quantized-level coefficients may be arranged in atwo-dimensional block form through inverse scanning. For the inversescanning, at least one of a diagonal up-right scan, a vertical scan, anda horizontal scan may be used.

The quantized-level coefficients may then be dequantized, then besecondary-inverse-transformed as necessary, and finally beprimary-inverse-transformed as necessary to generate a reconstructedresidual signal.

Inverse mapping in a dynamic range may be performed for a luma componentreconstructed through intra prediction or inter prediction beforein-loop filtering. The dynamic range may be divided into 16 equal piecesand the mapping function for each piece may be signaled. The mappingfunction may be signaled at a slice level or a tile group level. Aninverse mapping function for performing the inverse mapping may bederived on the basis of the mapping function. In-loop filtering,reference picture storage, and motion compensation are performed in aninverse mapped region, and a prediction block generated through interprediction is converted into a mapped region via mapping using themapping function, and then used for generating the reconstructed block.However, since the intra prediction is performed in the mapped region,the prediction block generated via the intra prediction may be used forgenerating the reconstructed block without mapping/inverse mapping.

When the current block is a residual block of a chroma component, theresidual block may be converted into an inverse mapped region byperforming scaling on the chroma component of the mapped region. Theavailability of the scaling may be signaled at the slice level or thetile group level. The scaling may be applied only when the mapping forthe luma component is available and the division of the luma componentand the division of the chroma component follow the same tree structure.The scaling may be performed on the basis of an average of sample valuesof a luma prediction block corresponding to the color difference block.In this case, when the current block uses inter prediction, the lumaprediction block may mean a mapped luma prediction block. A valuenecessary for the scaling may be derived by referring to a lookup tableusing an index of a piece to which an average of sample values of a lumaprediction block belongs. Finally, by scaling the residual block usingthe derived value, the residual block may be switched to the inversemapped region. Then, chroma component block restoration, intraprediction, inter prediction, in-loop filtering, and reference picturestorage may be performed in the inverse mapped area.

Information indicating whether the mapping/inverse mapping of the lumacomponent and chroma component is available may be signaled through aset of sequence parameters.

The prediction block of the current block may be generated on the basisof a block vector indicating a displacement between the current blockand the reference block in the current picture. In this way, aprediction mode for generating a prediction block with reference to thecurrent picture is referred to as an intra block copy (IBC) mode. TheIBC mode may be applied to M×N (M<=64, N<=64) coding units. The IBC modemay include a skip mode, a merge mode, an AMVP mode, and the like. Inthe case of a skip mode or a merge mode, a merge candidate list isconstructed, and the merge index is signaled so that one merge candidatemay be specified. The block vector of the specified merge candidate maybe used as a block vector of the current block. The merge candidate listmay include at least one of a spatial candidate, a history-basedcandidate, a candidate based on an average of two candidates, and azero-merge candidate. In the case of an AMVP mode, the difference blockvector may be signaled. In addition, the prediction block vector may bederived from the left neighboring block and the upper neighboring blockof the current block. The index on which neighboring block to use may besignaled. The prediction block in the IBC mode is included in thecurrent CTU or the left CTU and limited to a block in the alreadyreconstructed area. For example, a value of the block vector may belimited such that the prediction block of the current block ispositioned in an area of three 64×64 blocks preceding the 64×64 block towhich the current block belongs in the coding/decoding order. Bylimiting the value of the block vector in this way, memory consumptionand device complexity according to the IBC mode implementation may bereduced.

Syntax elements generated as a result of video coding/decoding areconverted into a bitstream through CABAC. Arithmetic coding canrepresent all occurrence bins of each syntax as a single real value in acertain probability interval.

In order to improve the performance of CABAC, it is necessary to use theoccurrence probability of each of binary symbols (bins) that are asaccurate as possible in arithmetic coding. The occurrence probability ofeach symbol is initialized to a specific value on the basis of thestatistical characteristics of each bin, and the occurrence probabilityof each bin is continuously updated by reflecting the occurrence resultof each bin while performing arithmetic coding, thereby continuouslyreflecting the time-varying statistical characteristics.

Therefore, the updating of the occurrence probability of each bin moreaccurately may be an important factor in the performance of the CABAC.That is, it is necessary to converge to a probability that quicklyreflects the actual statistical characteristic to be currently codedfrom the initial probability value while performing the arithmeticcoding using the initialized probability value. While it is necessary torapidly converge at a reasonable speed while maintaining the stabilityof the probability update as precisely as possible, it is impossible tosimultaneously guarantee the rapid update and the stability because onlyone probability update model is used in the related art. In addition, itis difficult to quickly reflect a change in probability occurring in themiddle of entropy coding.

The video coding/decoding method and apparatus according to the presentinvention may mix two or more probability update models having differentcharacteristics to maintain the stability and enable rapidly convergingto a certain probability. In addition, by ensuring both speed andstability, and by adjusting the probability update rate adaptivelyaccording to the occurrence status of past bins, it is possible toappropriately cope with a change in probability occurring in the middleof entropy coding.

The video coding/decoding method and apparatus according to the presentinvention may perform a context modeling step, a probability updatestep, and/or a probability interval determining step.

Hereinafter, the context modeling step will be described later.

In the context modeling step, context modeling for the current symbol isperformed using context initialization, adaptive context modelselection, and/or context model storage and synchronization process uponperforming the CABAC entropy coding for each syntax.

Hereinafter, the context initialization process will be described below.

In context modeling for the current symbol, context initialization maybe performed. The context initialization may be performed on the firstsyntax element (symbol) in each unit, and the initial probability forthe corresponding symbol may be set.

According to an embodiment, the context modeling initialization isperformed in at least one unit of picture (frame), slice, tile, CTUline, CTU, CU, and predetermined block size as in the example of FIG. 8.Herein, the predetermined block size may include one of the coding unitsdescribed in FIG. 3. FIG. 8 is a diagram illustrating a unit in whichthe context modeling initialization according to an embodiment of thepresent invention is performed.

For example, the context modeling may be initiated on a per CUT linebasis in horizontal direction as shown in FIG. 8 (c).

As another example, the context modeling may be initiated on a per CUTline basis in vertical direction.

As another example, the context modeling may be initiated on a per tilebasis in horizontal direction and/or vertical direction as shown in FIG.8 (b).

As another example, the context modeling may be initiated on a per CUTbasis in horizontal direction and/or vertical direction as shown in FIG.8 (d).

As another example, the context modeling may be initiated for each firstCTU in the CTU line on a per CUT line basis in horizontal direction, asshown in FIG. 8 (c).

Meanwhile, referring to FIG. 8 (c), the CTU line may mean a set of CTUsconfigured with at least one CTU in a horizontal direction or a rowdirection within a predetermined picture, slice, and tile. Also, onepicture, slice, and tile may include at least one or more identical CTUlines. Here, the slice may mean one of at least one slice included inone tile. Also, the tile may mean one of at least one tile included inone slice.

Also, referring to FIG. 8 (c), each of at least one or more CTU linesincluded in one picture, slice, and tile may have the same horizontalsize (or width). The horizontal size (or width) of each of the CTU linesmay be equal to the horizontal size (or width) of the one picture,slice, and tile. In addition, referring to FIG. 8 (c), the vertical size(or height) of each of at least one or more CTU lines included in onepicture, slice, and tile may be the same.

For example, one slice may include a plurality of CTU lines, in whichthe horizontal size of the first CTU line and the second CTU lineincluded in the plurality of CTU lines is equal to the horizontal sizeof the slice, and the horizontal size and the vertical size of the firstCTU line are equal to the horizontal size and the vertical size of thesecond CTU line, respectively.

As another example, one tile may include a plurality of CTU lines, inwhich the horizontal size of the third CTU line and the fourth CTU lineincluded in the plurality of CTU lines is equal to the horizontal sizeof the tile, and the horizontal size and the vertical size of the thirdCTU line are equal to the horizontal size and the vertical size of thefourth CTU line, respectively.

As another example, one tile may include at least one or more slices, inwhich one of the at least one or more slices may include a plurality ofCTU lines. The horizontal size of the first CTU line and the second CTUline included in the plurality of CTU lines is equal to the horizontalsize of the slice, and the horizontal size and the vertical size of thefirst CTU line are equal to the horizontal size and the vertical size ofthe second CTU line, respectively.

As another example, one slice may include at least one or more tiles,and one of the at least one or more tiles may include a plurality of CTUlines. The horizontal size of the first CTU line and the second CTU lineincluded in the plurality of CTU lines are equal to the horizontal sizeof the tile, and the horizontal size and the vertical size of the firstCTU line are equal to the horizontal size and the vertical size of thesecond CTU line, respectively.

According to an embodiment, the context modeling initialization unit mayvary for each predetermined unit.

For example, it is possible to adaptively select the contextinitialization unit for each picture as at least one unit of a slice, atile, a CTU line, a CTU, a CU, and a predetermined block size.

As another example, it is possible to adaptively select the contextinitialization unit for each slice as at least one unit of a tile, a CTUline, a CTU, a CU, and a predetermined block size.

As another example, it is possible to adaptively select the contextinitialization unit for each tile as at least one unit of a CTU line, aCTU, a CU, and a predetermined block size.

As another example, it is possible to adaptively select the contextinitialization unit for each CTU line as at least one unit of a CTU, aCU, and a predetermined block size.

Also, according to an embodiment, index information indicating aselected context modeling initialization unit may be transmitted (e.g.,context init_unit_idx).

For example, index information indicating a context modelinginitialization unit selected as at least one unit of a slice, a tile, aCTU line, a CTU, a CU, and a predetermined block size may be transmittedfor each unit.

Also, according to an embodiment, the initial setting of the contextmodel probability (LPS, MPS) for each syntax may vary using at least oneof syntax, a quantization parameter (QP), a slice type, and predictionmode.

For example, LPS or MPS according to the QP may be obtained by a linearequation for the QP and the occurrence probability. The linear equationis equal to an example of Equation 1.

LPS=QP×x+y  [Equation 1]

In Equation 1, x and y may be different for each context model.

As another example, LPS or MPS according to the QP may be obtained by anonlinear equation for the QP and the occurrence probability.

As another example, it is possible to have different initialprobabilities according to slice type (I-SLICE, P-SLICE, B-SLICE).

As another example, it is possible to have different initialprobabilities according to the prediction mode (inter prediction, intraprediction).

As another example, it is possible to have different initialprobabilities according to the prediction mode and the slice type.

As another example, according to slice type, x and y of the aboveequation representing the linear model of QP and the initial probabilitymay vary.

As another example, according to prediction mode, x and y of the aboveequation representing the linear model of QP and the initial probabilitymay vary.

Hereinafter, the adaptive context model selection process will bedescribed below.

In context modeling for the current symbol, adaptive context modelselection may be performed. N (positive integer) probability models maybe used for each syntax, and one of N probability models may be selectedaccording to the prediction information and/or syntax element of thecurrent block and the neighboring block.

Herein, the neighboring block may be a spatial/temporal adjacent blockof the current block.

For example, upon selecting a probability model for a syntax element ofthe current block, different probability models may be selectedaccording to a corresponding syntax of the neighboring block.

As another example, upon selecting the probability model for the syntaxelement of the current block, different probability models may beselected according to the prediction mode of the neighboring block.

As another example, upon selecting the probability model for the syntaxelement of the current block, different probability models may beselected according to the similarity of the motion information of thecurrent block and the neighboring block.

As another example, upon selecting the probability model for the syntaxelement of the current block, different probability models may beselected according to whether intra prediction modes of the currentblock and the neighboring block coincide with each other.

As another example, upon selecting the probability model for the syntaxelement of the current block, the probability model of the neighboringblock may be selected.

As another example, adaptive context model selection may be performed incontext modeling for the current symbol, and some probability models ofN (positive integer) probability models may be used for each syntax.

Meanwhile, the syntax element may include a coding parameter. Inaddition, the prediction mode may be information indicating a mode thatis coded/decoded in intra prediction (or an intra prediction mode), amode that is coded/decoded in inter prediction (or an inter predictionmode), or an IBC mode. In addition, the neighboring block may include aleft adjacent block to an upper adjacent block of the current block.

According to an embodiment, upon selecting the probability model for thesyntax element of the current block, different probability models may beselected according to whether the prediction mode of the neighboringblock is an intra prediction mode or not.

For example, upon selecting the probability model for the predictionmode of the current block, different probability models may be selectedaccording to whether the prediction mode of the neighboring block is anintra prediction mode or not.

As another example, upon selecting a probability model for theprediction mode of the current block, another probability model may beselected according to whether the prediction mode of the left adjacentblock of the current block is an intra prediction mode or not.

As another example, upon selecting a probability model for theprediction mode of the current block, different probability models maybe selected according to whether the prediction mode of the upperadjacent block of the current block is an intra prediction mode or not.

As another example, the context index for the prediction mode of thecurrent block may be determined according to whether the prediction modeof the left adjacent block of the current block is an intra predictionmode.

As another example, the context index for the prediction mode of thecurrent block may be determined according to whether the prediction modeof the upper adjacent block of the current block is an intra predictionmode.

In addition, according to an embodiment, upon selecting the probabilitymodel for the syntax element of the current block, different probabilitymodels may be selected according to whether the prediction mode of theneighboring block is the inter prediction mode. For example, the syntaxelement may include an indicator indicating whether the inter predictionmode for the current block is an affine inter mode or a sub-block mergemode.

For example, upon selecting the probability model for the predictionmode of the current block, different probability models may selectedaccording to whether the prediction mode of the left adjacent block ofthe current block is an affine inter mode and/or a sub-block merge mode.

As another example, upon selecting the probability model for theprediction mode of the current block, different probability models mayselected according to whether the prediction mode of the upper adjacentblock of the current block is an affine inter mode and/or a sub-blockmerge mode.

According to an embodiment, upon selecting the probability model for asyntax element of the current block, different probability models may beselected according to whether the prediction mode of the neighboringblock is an IBC mode or not.

For example, upon selecting the probability model for the predictionmode of the current block of the current block, different probabilitymodels may be selected according to whether the prediction mode of theneighboring block is an IBC mode or not.

As another example, upon selecting the probability model for theprediction mode of the current block, different probability models maybe selected according to whether the prediction mode of the leftadjacent block of the current block is an IBC mode or not.

As another example, upon selecting the probability model for theprediction mode of the current block, different probability models maybe selected according to whether the prediction mode of the upperadjacent block of the current block is an IBC mode or not.

As another example, the context index for the prediction mode of thecurrent block may be determined according to whether the prediction modeof the left adjacent block of the current block is an IBC mode or not

As another example, the context index for the prediction mode of thecurrent block may be determined according to whether the prediction modeof the upper adjacent block of the current block is an IBC mode or not.

Further, according to an embodiment, upon selecting the probabilitymodel for the syntax element of the current block, a probability modelof a left adjacent block or an upper adjacent block of the current blockmay be selected. For example, the syntax element may include anindicator indicating whether the intra prediction type for the currentblock is a matrix weighted intra prediction (MIP) or not. Also, forexample, the intra prediction type for the current block may mean anintra prediction type for luma samples included in the current block (ora luma component block of the current block).

Meanwhile, in the current block having a size of W×H (horizontalsize×vertical size), the MIP may predict samples of the current block onthe basis of one line of H reconstructed neighboring boundary samples onthe left side of the current block and/or one line of W reconstructedneighboring boundary samples on the upper side of the current block.

Further, according to an embodiment, upon selecting the probabilitymodel for the syntax element of the current block, different probabilitymodels may be selected according to syntax of the neighboring block ofthe current block. For example, the syntax element may include anindicator indicating whether the prediction mode of the current block isa planar mode. Also, for example, the prediction mode of the currentblock may mean a prediction mode of luma samples included in the currentblock (or a luma component block of the current block).

For example, upon selecting the probability model for the syntax elementof the current block, different probability models may be selectedaccording to whether the prediction mode of the left adjacent block orthe upper adjacent block of the current block is a planar mode or not.

As another example, upon selecting the probability model for the syntaxelement of the current block, another probability model may be selectedaccording to whether the prediction modes of the current block and theleft adjacent block or the upper adjacent block of the current block area Planar mode or not. Meanwhile, the planar mode may be determined onthe basis of a syntax element indicating whether the mode is a planarmode.

As another example, upon selecting the probability model for the syntaxelement of the current block, a probability model of a left adjacentblock or an upper adjacent block of the current block may be selected.

Hereinafter, the process of storing and synchronizing the context modelwill be described below.

In context modeling for the current symbol, context model storage and/orsynchronization may be performed.

The probability model storage and/or synchronization may be performed inat least one unit of a picture, a slice, a tile, a CTU line, a CTU, aCU, and a predetermined block size.

According to an embodiment, the probability information (LPS, MPS)calculated for each syntax on a per slice basis may be stored.

Also, according to an embodiment, probability information calculated foreach syntax on a per CTU basis may be stored.

Also, according to an embodiment, entropy coding is performed on a basisof the probability of the spatial neighborhood and on a per slice basis,and the context model of the current slice is synchronized using atleast one probability information stored in temporal/spatialneighborhood when performing the context modeling initialization.

Herein, the probability information of the spatial neighborhood meansthe probability information stored before the current unit in the samepicture, and the probability information of the temporal neighborhoodmeans the probability information in the same position as the currentunit or the neighboring position thereof in the picture where decodingis performed before the current picture.

FIG. 9 is a diagram illustrating neighboring blocks of a current blockaccording to an embodiment of the present invention.

Referring to FIG. 9, an example of a spatial neighboring position and atemporal neighboring position of CTU unit is shown. “X” is a CTU to becurrently synchronized, “a”, “b”, “c”, and “d” are spatial neighboringpositions, and “e”, “f”, “g”, “h”, “i”, “j”, “k”, “1”, and “m” meantemporal neighboring positions existing within a temporal neighboringpicture. Also, the picture where coding/decoding is currently performedis an N^(th) picture, and the temporal neighboring picture may be anN−i^(th) picture where decoding is performed at an i-th earlier picture.Meanwhile, there may be one or more temporal neighboring pictures.Further, the spatial neighboring position and/or the temporalneighboring position to be referred to may be one or more.

Also, according to an embodiment, the units of probability model storageand synchronization may be different from each other.

For example, the probability information calculated for each syntax maybe stored on a per CTU line basis and synchronized on a per CTU basis.

As another example, the probability information calculated for eachsyntax may be stored on a per slice basis and synchronized on a per CUbasis.

As another example, the probability information calculated for eachsyntax may be stored on a per tile basis and synchronized on a per slicebasis.

As another example, when the probability information calculated for eachsyntax is stored on a per CTU line basis and is synchronized on a perCTU basis, each CTU may be synchronized using the probabilityinformation stored in the CTU line to which the CTU belongs.

Hereinafter, the probability update step will be described later.

In a probability update step, upon performing the CABAC entropy codingfor each syntax element, the probability update may be performed byusing at least one of a table-based probability update, anoperation-based probability update, a momentum-based probability update,a multiple probability update, and a boundary-based probability update.

Meanwhile, the least probable state (LPS) may be 0 or 1, and may be setto a state having a case where the occurrence probability exceeds 0.5 inan adaptive manner or fixedly set to one state.

Also, the most probable state (MPS) may be set to 0 or 1, and may be setto a state having a case where the occurrence probability is less than0.5 in an adaptive manner or fixedly set to one state.

Hereinafter, the table-based probability update process will bedescribed later.

In performing CABAC probability update for each syntax element, thetable-based probability update may be performed.

According to an embodiment, the probability table may be constructed byquantizing the probability range to a positive integer N.

For example, the probability table may be constructed by quantizing theprobability range of real number of 0 to 1 to 256.

As another example, the probability table may be constructed byquantizing the probability range of real number of 0 to 0.5 to 64.

Meanwhile, the probability range of real number may be scaled to aninteger for integer operations.

As another example, the probability table may be constructed byquantizing even same probability range of real number to differentnumbers. For example, the probability table may be constructed byquantizing the real probability range of 0 to 1 to 64 or 256.

As another example, in the probability range of real number of 0 to 1,the probability range of real number between 0 and 0.5 may be quantizedto 256 to construct the probability table, and the probability range ofreal number between 0.5 and 1 is 64 is quantized to 64 to construct theprobability table.

Also, according to an embodiment, the probability update may beperformed through a linear or nonlinear equation capable of deriving theprobability index updated according to the current probability index,and the occurrence of LPS and MPS.

Also, according to an embodiment, a conversion table may be constructedusing an index indicating the current probability in the configuredprobability table, and an index to be converted according to theoccurrence of LPS and MPS. The probability table and the probabilityconversion table may be used to perform the probability update accordingto the symbol occurrence state (LPS, MPS).

Meanwhile, the type of the table may vary depending on the syntaxelement.

Hereinafter, the operation-based probability update process will bedescribed below.

In performing the CABAC probability update for each syntax element, anoperation-based probability update may be performed.

According to an embodiment, the probability update may be performedthrough an operation of an example of Equation 2 at the occurrence timeof LPS. P_(old) is the current probability, and P_(new) is the updatedprobability.

$\begin{matrix}{P_{new} = {P_{old} + \frac{1 - P_{old}}{M}}} & \left\lbrack {{Equation}\mspace{14mu} 2} \right\rbrack\end{matrix}$

Also, according to an embodiment, the probability update may beperformed through an operation of an example of Equation 3 at theoccurrence time of MPS. P_(old) is the current probability, and P_(new)is the updated probability.

$\begin{matrix}{P_{new} = {P_{old} - \frac{P_{old}}{M}}} & \left\lbrack {{Equation}\mspace{14mu} 3} \right\rbrack\end{matrix}$

Also, according to an embodiment, in Equations 2 and 3, M has a valuegreater than 1 and may be applied to a probability update operation oneach syntax element. Also, M may mean a probability update rate, and thesize of the M value and the probability update rate may be inverselyproportional to each other.

As an example, M in the probability update operation on all syntaxelements may be fixed to 128.

As another example, M in the probability update operation for all syntaxelements may be set to a value with optimal cost.

Meanwhile, the cost used to find the optimal M for all syntax elementsmay be a bitrate. In addition, the optimal M for all syntax elements maybe transmitted in at least one unit of picture, slice, tile, CTU line,and CTU.

As another example, M in the probability update operation on each syntaxelement may be set differently, and M may have one value of 16, 32, 64,128, and 256.

As another example, M in the probability update operation on each syntaxelement may be set to a value having an optimal cost.

Meanwhile, the cost used to find the optimal M for each syntax elementmay be a bitrate. In addition, optimal M for each syntax element may betransmitted in at least one unit of picture, slice, tile, CTU line, andCTU.

As another example, M may be set to at least one unit of picture(frame), slice, tile, CTU line, CTU, CU, and a predetermined block size.

As another example, the value of M may vary according to LPS and MPS.

As another example, the value of M may vary according to a syntaxelement.

Hereinafter, a momentum-based probability update process will bedescribed later.

In performing the CABAC probability update for each syntax element, amomentum-based probability update process may be performed.

According to an embodiment, K past occurrence symbols may be stored foreach syntax element, and the momentum-based probability update may beperformed according to the current occurrence symbols (bins) and thestored past symbols.

Meanwhile, the number of past occurrence symbols stored according toeach syntax element may be different from each other.

Also, according to an embodiment, the momentum-based probability updatemay be performed in the table-based probability update.

For example, when the current occurrence symbol is LPS and the number ofLPSs among the past symbols is larger than a certain threshold value,the probability update may be performed by adding an integer m to theupdated probability index.

As another example, when the current occurrence symbol is LPS and thenumber of MPSs among the past symbols is larger than a certain thresholdvalue, the probability update may be performed by subtracting an integerm from the updated probability index.

As another example, when the current occurrence symbol is the MPS andthe number of LPSs among the past symbols is larger than a certainthreshold value, the probability update may be performed by subtractingan integer m from the updated probability index.

As another example, when the current occurrence symbol is the MPS andthe number of MPSs among past symbols is larger than a certain thresholdvalue, the probability update may be performed by adding an integer m tothe updated probability index.

Also, according to an embodiment, an momentum-based probability updatemay be performed in an operation-based probability update.

For example, when the current occurrence symbol is LPS and the number ofLPSs among the past symbols is higher than a certain threshold value,the probability update may be performed by multiplying a real number nand a parameter M of the update model.

As another example, when the current occurrence symbol is the LPS andthe number of MPSs among past symbols is larger than a certain thresholdvalue, the probability update may be performed by dividing a parameter Mof the update model by a real number n.

As another example, when the current occurrence symbol is the MPS andthe number of LPSs among past symbols is larger than a certain thresholdvalue, the probability update may be performed by dividing a parameter Mof the update model by a real number n.

As another example, when the current occurrence symbol is the MPS andthe number of MPSs among the past symbols is larger than a certainthreshold value, the probability update may be performed by multiplyinga parameter M of the update model with a real number n.

As another example, the probability update may be performed using anexample of Equation 4. It is possible to increase or decrease the degreeof the existing probability update according to the equality between thepast occurrence symbol and the current occurrence symbol.

$\begin{matrix}{{P_{new} = {{P_{old} + {\Delta_{new}^{\prime}.\Delta_{new}^{\prime}}} = {\Delta_{new} \times {\prod\limits_{i = 1}^{K}\left( \frac{W + i + 1}{W + i} \right)^{\alpha_{i}}}}}}{\alpha_{i} = {{\left( {b_{0}==b_{i}} \right)?\mspace{14mu} 1}\text{:}\mspace{11mu} {–1}}}} & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack\end{matrix}$

In Equation 4, Δ_(new) may denote the degree of existing probabilityupdate. Further, W is a positive integer value, and bi may mean an i-thearlier past bin. The size of W and i and the probability updateincrement/decrement amount may be inversely proportional to each other.

As another example, the probability update may be performed using anexample of Equation 5. The probability update parameter M may beadjusted according to the occurrence of the past symbol and the currentsymbol.

$\begin{matrix}{{{{if}\mspace{14mu} {MPS}},{P_{new} = {P_{old} + \frac{1 - P_{old}}{M - a}}}}{{{else}\mspace{14mu} {if}\mspace{14mu} {LPS}},{P_{new} = {P_{old} - \frac{P_{old}}{M - a}}}}} & \left\lbrack {{Equation}\mspace{14mu} 5} \right\rbrack\end{matrix}$

Referring to Equation 5, when the number of cases that are same to thecurrent symbol among arbitrary K past symbols is higher than a certainthreshold value, a may have any positive integer (M>a).

Also, when the number of cases that are different from the currentsymbol among arbitrary K past symbols is higher than a certain thresholdvalue, a may have any negative integer.

Also, a value of the probability update parameter M may be adjustedaccording to the probability update number. For example, when the numberof probability update number for that symbol is higher than a certainthreshold value, a may have any positive integer. As another example,when the probability update number for the corresponding symbol ishigher than a certain threshold value, a may have any negative integer.

Hereinafter, a multiple probability update process will be describedlater.

In performing the CABAC probability update for each syntax element, amultiple probability update may be performed.

According to an embodiment, in performing the probability update on thecontext model of each syntax element, N (positive integer) probabilityupdate models may be used.

Herein, the probability update model may be one of the table-basedprobability update, the operation-based probability update, and themomentum-based probability update.

Further, according to an embodiment, the number and/or the type of theprobability update models may vary according to each syntax element.

Also, according to an embodiment, when the probability update model usedaccording to each syntax element is a table-based probability updatemodel, types of tables may vary.

Also, according to an embodiment, when the probability update model usedaccording to each syntax element is an operation-based probabilityupdate model, values of M may be different from each other.

Also, according to an embodiment, the initial probability values for theprobability update for each syntax element may be different from eachother.

As one example, two probability update models may be used for one syntaxelement, and the probabilities may be updated as an average of eachprobability update model.

As another example, two probability update models may be used for onesyntax element, in which the update is performed using the firstprobability model up to the K-th update, and thereafter the update isperformed using the second probability model.

As another example, two probability update models may be used for onesyntax element, in which the update is performed using one probabilitymodel up to the K-th update, and thereafter the update is performedusing an average of two probability models.

Meanwhile, the values of K may vary according to each syntax element.

As another example, three probability update models are used for onesyntax element, and the probabilities are updated using a weightedaverage of each update model.

As another example, three probability update models are used for onesyntax element, and the probability update is performed using an exampleof Equation 6.

$\begin{matrix}{{{if}\mspace{14mu} {MPS}},{{P\; 3_{new}} = {{P\; 3_{old}} + \frac{{P\; 1_{new}} - {P\; 3_{old}}}{M}}},{{if}\mspace{14mu} {LPS}},{{P\; 3_{new}} = {{P\; 3_{old}} - \frac{{P\; 3_{old}} - {P\; 2_{new}}}{M}}}} & \left\lbrack {{Equation}\mspace{14mu} 6} \right\rbrack\end{matrix}$

In Equation 6, the probability update is performed with P3, and P1 andP2 may be the operation-based probability update, the momentum basedprobability update, or the table-based probability update.

Hereinafter, the boundary-based probability updating process will bedescribed below.

Upon performing CABAC probability update on each syntax element, aboundary-based probability update may be performed.

According to an embodiment, the range of probability update for thecontext model of each syntax element may be initialized to N (positiveinteger) probability values.

As one example, initialization may be performed using one probabilityvalue. The first probability value may be the initial probability of thecorresponding context model, and the probability range boundary may beinitialized by adding a predetermined offset to the first probability.Herein, the predetermined offset may have one or two values.

As another example, the initialization may be performed using twoprobability values. The first probability value may be the maximumboundary of the probability update range, and the second probabilityvalue may be the minimum boundary. Here, the initial probability may bethe average probability of the first probability and the secondprobability.

As another example, the initialization may be performed using threeprobability values. The first probability value may be the maximumboundary of the probability update range, the second probability valuemay be the probability minimum boundary, and the third probability valuemay be the initial probability value of the context model.

As another example, the initialization may be performed using threeprobability values. The first probability value may be the maximumboundary of the probability update range, the second probability valuemay be the probability minimum boundary, and a weighted average of thefirst probability value, the second probability value, and the thirdprobability value may be an initial probability value of the contextmodel.

Also, according to an embodiment, the initialized probability updaterange (maximum boundary, minimum boundary) may be updated using theupdate method.

As an example, the probability update range may be updated with anoperation-based update method.

As another example, the probability update range may be updated usingthe table-based update method.

As another example, the probability update range may be updated usingthe momentum-based update method.

As another example, the maximum boundary and the minimum boundary may beupdated using update methods different from each other.

As another example, the maximum boundary and minimum boundary mayconverge to a certain probability as the update is performed.

Also, according to an embodiment, the initial probability value of thecontext model to be initialized may be updated using the probabilityupdate range and the update method.

As an example, the probability of the context model may be updated witha probability that is equal to or greater than/greater than the minimumboundary.

As another example, the probability of the context model may be updatedwith a probability that is less than or equal to/less than the maximumbounds.

As another example, the probability of the context model may be updatedwith a probability that is equal to or greater than/greater than theminimum boundary, and less than or equal to/less than the maximumboundary.

As another example, the probability of the context model may be updatedusing the operation-based update method within the predeterminedprobability range.

As another example, the probability of the context model may be updatedusing the momentum-based update method within the predeterminedprobability range.

For example, the probability update may be performed using an example ofEquation 7.

$\begin{matrix}{{{if}\mspace{14mu} {MPS}},{P_{new} = {P_{old} + \frac{P_{U\_ new} - P_{old}}{M}}},{{if}\mspace{14mu} {LPS}},{P_{new} = {P_{old} - \frac{P_{old} - P_{L\_ new}}{M}}}} & \left\lbrack {{Equation}\mspace{14mu} 7} \right\rbrack\end{matrix}$

In Equation 7, P_(old), P_(new) are probabilities before and afterupdating the context model, and P_(U_new), P_(L_new) are the updatedmaximum and minimum boundaries. M is a parameter indicating a updaterate of the operation-based update.

Hereinafter, the probability interval determination step will bedescribed later.

In the probability interval determination step, upon performing theCABAC entropy coding on each syntax element, the probability intervalmay be determined using at least one of a table-based probabilityinterval determination and an operation-based probability intervaldetermination.

Hereinafter, a table-based probability interval determination processwill be described later.

In determining a probability interval for each syntax element, atable-based probability interval determination may be performed.

According to an embodiment, the probability interval table may have atwo-dimensional table of [M]×[N] according to an occurrence probabilityindex having a positive integer M and a current probability intervalindex having a positive integer N.

For example, the occurrence probability index may be an LPS occurrenceprobability index.

As another example, the occurrence probability index may be an MPSoccurrence probability index.

Also, according to an embodiment, the element value of the table may bea probability.

As one example, the probability value may be between 0 and 0.5.

As another example, the probability value may be between 0.5 and 0.1.

As another example, the probability value may be between 0 and 1.

Further, according to an embodiment, the element probability value ofthe table may be represented by a positive integer of K-bits.

For example, a probability value between 0 and 0.5 may be represented asa positive integer between 0 and 255 (8 bits).

As another example, a probability value between 0 and 1 may berepresented as a positive integer between 0 and 512 (9 bits).

Hereinafter, an operation-based probability interval determinationprocess will be described below.

Upon determining a probability interval for each syntax element, anoperation-based probability interval determination may be performed.

According to an embodiment, an example of Equation 8 may be used indetermining the probability interval.

r _(new) =r _(old) ×p  [Equation 8]

In Equation 8, r_(old) is the previous probability interval, and r_(new)is the determined current probability interval. p is the occurrenceprobability.

As one example, p may be an LPS occurrence probability index.

As another example, p may be an MPS occurrence probability index.

Meanwhile, two probability intervals may be determined for one syntaxelement, and the probability interval determination is performed as anaverage of two probability intervals.

FIG. 10 is a flowchart illustrating a video decoding method according toan embodiment of the present invention.

Referring to FIG. 10, a bitstream including a predetermined syntaxelement may be obtained (S1001).

Further, at least one of a context model determination, a probabilityupdate, and a probability interval determination may be performed on apredetermined syntax element (S1002).

Meanwhile, the determination of the context model may include at leastone of a context initialization process, an adaptive context modelselection process, and a context model storage/synchronization process(i.e., a context model storage process and/or a context modelsynchronization process).

Meanwhile, the context initialization process may be performed in atleast one unit of a picture (frame), a slice, a tile, a CTU line, a CTU,a CU, and a predetermined block size.

Meanwhile, the context model storage/synchronization process may beperformed in at least one unit of a picture (frame), a slice, a tile, aCTU line, a CTU, a CU, and a predetermined block size.

Meanwhile, the probability update may include at least one of atable-based probability update, an operation-based probability update, amomentum-based probability update, a multiple probability update, and aboundary-based probability update.

Meanwhile, in the table-based probability update, the probability tablemay be constructed by quantizing the probability range as a positiveinteger.

Meanwhile, the momentum-based probability update may be performed on thebasis of the current occurrence symbol and the past occurrence symbol.

The probability interval determination may include at least one of atable-based probability interval determination and an operation-basedprobability interval determination.

Meanwhile, in the table-based probability interval determination, theprobability interval table may be represented by a two-dimensional tableof the occurrence probability index (positive integer M) and the currentprobability interval (positive integer N).

Further, a predetermined syntax element may be arithmetically decoded onthe basis of the performance result (S1003).

FIG. 11 is a flowchart illustrating an video coding method according toan embodiment of the present invention.

Referring to FIG. 11, at least one of a context model determination, aprobability update, and a probability interval determination may beperformed on a predetermined syntax element (S1101).

Further, a predetermined syntax element may be arithmetically coded onthe basis of the performance result (S1102).

Meanwhile, the context model determination may include at least one of acontext initialization process, an adaptive context model selectionprocess, and a context model storage/synchronization process.

Meanwhile, the context initialization process may be performed in atleast one unit of a picture (frame), a slice, a tile, a CTU line, a CTU,a CU, and a predetermined block size.

Meanwhile, the storage/synchronization process of the context model maybe performed in at least one unit of a picture (frame), a slice, a tile,a CTU line, a CTU, a CU, and a predetermined block size.

Meanwhile, the probability update may include at least one of atable-based probability update, an operation-based probability update, amomentum-based probability update, a multiple probability update, and aboundary-based probability update.

Meanwhile, in the table-based probability update, the probability tablemay be constructed by quantizing the probability range as a positiveinteger.

Meanwhile, the momentum-based probability update may be performed on thebasis of the current occurrence symbol and the past occurrence symbol.

Meanwhile, the determination of the probability interval may include atleast one of a table-based probability interval determination and anoperation-based probability interval determination.

Meanwhile, in the table-based probability interval determination, theprobability interval table may be represented by a two-dimensional tableof the occurrence probability index (positive integer M) and the currentprobability interval (positive integer N).

Furthermore, a bitstream including a predetermined syntax element thatis arithmetically coded may be generated (S1103).

According to the present invention, video coding/decoding method andapparatus with improved compression efficiency may be provided.

Also, according to the present invention, a video coding/decoding methodand apparatus using CABAC with improved compression efficiency may beprovided.

Also, according to the present invention, a recording medium storing abitstream generated by the video coding/decoding method or apparatus ofthe present invention may be provided.

Also, according to the present invention, a method and apparatus forentropy coding/context adaptive binary arithmetic binary arithmeticcoding (CABAC) for various types of binary information such asprediction information, transform coefficients, and signalinginformation generated in a video encoding/decoding process may beprovided.

Also, according to the present invention, a video coding/decoding methodand apparatus that maintains the stability by mixing two or moreprobability update models having different characteristics and convergesto the occurrence probability of each bin at an appropriate rate may beprovided.

Also, according to the present invention, a video coding/decoding methodand apparatus that appropriately adjusts a probability update rateaccording to the occurrence state of past bins can be provided.

The above embodiments may be performed in the same method in an encoderand a decoder.

At least one or a combination of the above embodiments may be used toencode/decode a video.

A sequence of applying to above embodiment may be different between anencoder and a decoder, or the sequence applying to above embodiment maybe the same in the encoder and the decoder.

The above embodiment may be performed on each luma signal and chromasignal, or the above embodiment may be identically performed on luma andchroma signals.

A block form to which the above embodiments of the present invention areapplied may have a square form or a non-square form.

The above embodiment of the present invention may be applied dependingon a size of at least one of a coding block, a prediction block, atransform block, a block, a current block, a coding unit, a predictionunit, a transform unit, a unit, and a current unit. Herein, the size maybe defined as a minimum size or maximum size or both so that the aboveembodiments are applied, or may be defined as a fixed size to which theabove embodiment is applied. In addition, in the above embodiments, afirst embodiment may be applied to a first size, and a second embodimentmay be applied to a second size. In other words, the above embodimentsmay be applied in combination depending on a size. In addition, theabove embodiments may be applied when a size is equal to or greater thata minimum size and equal to or smaller than a maximum size. In otherwords, the above embodiments may be applied when a block size isincluded within a certain range.

For example, the above embodiments may be applied when a size of currentblock is 8×8 or greater. For example, the above embodiments may beapplied when a size of current block is 4×4 or greater. For example, theabove embodiments may be applied when a size of current block is 16×16or greater. For example, the above embodiments may be applied when asize of current block is equal to or greater than 16×16 and equal to orsmaller than 64×64.

The above embodiments of the present invention may be applied dependingon a temporal layer. In order to identify a temporal layer to which theabove embodiments may be applied, a corresponding identifier may besignaled, and the above embodiments may be applied to a specifiedtemporal layer identified by the corresponding identifier. Herein, theidentifier may be defined as the lowest layer or the highest layer orboth to which the above embodiment may be applied, or may be defined toindicate a specific layer to which the embodiment is applied. Inaddition, a fixed temporal layer to which the embodiment is applied maybe defined.

For example, the above embodiments may be applied when a temporal layerof a current image is the lowest layer. For example, the aboveembodiments may be applied when a temporal layer identifier of a currentimage is 1. For example, the above embodiments may be applied when atemporal layer of a current image is the highest layer.

A slice type or a tile group type to which the above embodiments of thepresent invention are applied may be defined, and the above embodimentsmay be applied depending on the corresponding slice type or tile grouptype.

In the above-described embodiments, the methods are described based onthe flowcharts with a series of steps or units, but the presentinvention is not limited to the order of the steps, and rather, somesteps may be performed simultaneously or in different order with othersteps. In addition, it should be appreciated by one of ordinary skill inthe art that the steps in the flowcharts do not exclude each other andthat other steps may be added to the flowcharts or some of the steps maybe deleted from the flowcharts without influencing the scope of thepresent invention.

The embodiments include various aspects of examples. All possiblecombinations for various aspects may not be described, but those skilledin the art will be able to recognize different combinations.Accordingly, the present invention may include all replacements,modifications, and changes within the scope of the claims.

The embodiments of the present invention may be implemented in a form ofprogram instructions, which are executable by various computercomponents, and recorded in a computer-readable recording medium. Thecomputer-readable recording medium may include stand-alone or acombination of program instructions, data files, data structures, etc.The program instructions recorded in the computer-readable recordingmedium may be specially designed and constructed for the presentinvention, or well-known to a person of ordinary skilled in computersoftware technology field. Examples of the computer-readable recordingmedium include magnetic recording media such as hard disks, floppydisks, and magnetic tapes; optical data storage media such as CD-ROMs orDVD-ROMs; magneto-optimum media such as floptical disks; and hardwaredevices, such as read-only memory (ROM), random-access memory (RAM),flash memory, etc., which are particularly structured to store andimplement the program instruction. Examples of the program instructionsinclude not only a mechanical language code formatted by a compiler butalso a high level language code that may be implemented by a computerusing an interpreter. The hardware devices may be configured to beoperated by one or more software modules or vice versa to conduct theprocesses according to the present invention.

Although the present invention has been described in terms of specificitems such as detailed elements as well as the limited embodiments andthe drawings, they are only provided to help more general understandingof the invention, and the present invention is not limited to the aboveembodiments. It will be appreciated by those skilled in the art to whichthe present invention pertains that various modifications and changesmay be made from the above description.

Therefore, the spirit of the present invention shall not be limited tothe above-described embodiments, and the entire scope of the appendedclaims and their equivalents will fall within the scope and spirit ofthe invention.

INDUSTRIAL APPLICABILITY

The present invention may be used when performing imageencoding/decoding.

1. A video decoding method, comprising: acquiring a bitstream includinga context element; performing at least one of a context modeldetermination, a probability update, and a probability intervaldetermination on the syntax element; and arithmetically decoding thepredetermined syntax element on the basis of a result of theperformance. 2-20. (canceled)
 21. The video decoding method of claim 1,wherein the context model determination includes a contextinitialization, the context initialization is performed on the firstCoding Tree Unit (CTU) of a horizontal line of CTUs, and the contextinitialization is performed using information of a block which isadjacent to an upper side of the first CTU.
 22. The video decodingmethod of claim 21, wherein the context initialization is performed onthe first CTU of the horizontal line of the CTUs in a tile.
 23. Thevideo decoding method of claim 1, wherein the context modeldetermination comprises to select a probability model for the syntaxelement.
 24. The video decoding method of claim 23, wherein theprobability model for the syntax element of a current block isdetermined based on a syntax element of an adjacent block of the currentblock which corresponds to the syntax element of the current block. 25.The video decoding method of claim 24, wherein the adjacent blockcomprises a block which is adjacent to a left side of the current blockand a block which is adjacent to an upper side of the current block. 26.The video decoding method of claim 23, wherein the probability model forthe syntax element of a current block is determined based on aprediction mode of an adjacent block of the current block.
 27. The videodecoding method of claim 26, wherein the probability model for thesyntax element of the current block is determined based on whether theprediction mode of the adjacent block is an intra prediction mode. 28.The video decoding method of claim 26, wherein the probability model forthe syntax element of the current block is determined based on whetherthe prediction mode of the adjacent block is an affine inter mode or asub-block merge mode.
 29. The video decoding method of claim 26, whereinthe probability model for the syntax element of the current block isdetermined based on whether the prediction mode of the adjacent block isan intra block copy mode.
 30. The video decoding method of claim 26,wherein the probability model for the syntax element of the currentblock is determined based on whether the prediction mode of the adjacentblock is a matrix weighted intra prediction mode.
 31. The video decodingmethod of claim 26, wherein the probability model for the syntax elementof the current block is determined based on an indicator which is usedto determine whether a planar mode is used for the adjacent block.
 32. Avideo encoding method, comprising: performing at least one of a contextmodel determination, a probability update, and a probability intervaldetermination on a syntax element; arithmetically decoding thepredetermined syntax element on the basis of a result of theperformance; and generating a bitstream including the syntax elementarithmetically decoded.
 33. The video encoding method of claim 32,wherein the context model determination includes a contextinitialization, the context initialization is performed on the firstCoding Tree Unit (CTU) of a horizontal line of CTUs, and the contextinitialization is performed using information of a block which isadjacent to an upper side of the first CTU.
 34. The video encodingmethod of claim 32, wherein the context model determination comprises toselect a probability model for the syntax element.
 35. The videoencoding method of claim 34, wherein the probability model for thesyntax element of a current block is determined based on a syntaxelement of an adjacent block of the current block which corresponds tothe syntax element of the current block.
 36. A non-transitorycomputer-readable medium storing the bitstream generated by the videoencoding method of claim
 33. 37. A non-transitory computer-readablemedium storing a bitstream, the bitstream comprising: a syntax element;wherein at least one of a context model determination, a probabilityupdate, and a probability interval determination on the predeterminedsyntax element is performed; and the predetermined syntax element isarithmetically decoded on the basis of a result of the performance. 38.The non-transitory computer-readable medium of claim 37, wherein thecontext model determination includes a context initialization, thecontext initialization is performed on the first Coding Tree Unit (CTU)of a horizontal line of CTUs, and the context initialization isperformed using information of a block which is adjacent to an upperside of the first CTU.
 39. The non-transitory computer-readable mediumof claim 37, wherein the context model determination comprises to selecta probability model for the syntax element.